G-d wants something to happen and it happens. So how could I possibly choose to do something He doesn’t like? Who’s more powerful, after all?
The Short Answer:
True, His knowledge is different from that of any of His creations. We are conscious only of that which already exists. His consciousness, on the other hand, becomes reality, as it congeals and manifests as a physical world.
However, by His will, Man is given free choice in certain areas. Which means that G-d makes an exception in those instances: He withholds His consciousness from its descent into reality -- since that would force a person to behave according to His will.
A Little Longer Answer:
Now we're getting a little more challenging. After all, G-d is the One "who speaks and it comes to be." "All that the Eternal desires, He does," the Psalmist sings. So how could we little creatures have the chutzpah to believe we can possibly do something against His will?
The answer, elucidated by the masters of Chabad Chassidism, is as follows: Before G-d made the world, He chose certain activities He will like people to do, and others He will not like. It was entirely free choice on His part -- He could have just as well chosen to enjoy stealing, and then made a world where stealing makes sense. But He didn't. He chose that stealing would be something He would abhor. Along with a whole slew of other things. And He chose that charitable acts, on the other hand, would chalk up major scores. 
Then He made a world where people could decide which sort of activities they would choose. They could choose to steal, abuse, corrupt, etc., and get Him real upset. Or they could choose acts of kindness for which He would express His pleasure.
To create even more excitement, He made sure this world would put up a decent fight with those trying to do His will, carefully balancing matters so that anyone with sufficient resolve would eventually come out the winner. In fact, the challenges themselves are only meant to bring out the inner powers of the human protagonists. Complex, true. But you've got to admit, it's a lot more exciting than watching a planet of robots acting out their factory default settings.
Of course, most things do run just as directed. The stars in the sky, the plants growing on the ground, the animals and even the ministering angels all have no say whatsoever in how they will behave and how much responsibility they will take. Only human beings. And even the free choice of human beings is limited: Once G-d says something is going to happen, it's going to happen. He decides an awful lot about your genetic structure, and you didn't choose your parents, either. But there's still plenty G-d just keeps to Himself and never decrees, only waiting for you to make that choice that it should happen.
Kabbalists talk of this as the difference between G-d's thoughts and G-d's speech: Speech is when He wills something to happen, and that will descends into this world and causes it to be. Thought is when He wills something, and waits for it to be. G-d said, "There should be light!" -- and there was light. G-d saw -- meaning "thought about" (Nachmanides on this verse) -- the light, that is was good, and He distinguished between the light and the darkness. From then on, He preferred a state where good (light) dominates and bad (darkness) is subjugated. And He waits for that to happen.
Before you are born, the Talmud tells, a ministering angel gently carries your fetal form before the Throne of Glory and asks what will be with you. G-d decrees many things -- your talents, your personality, your state of health, your monetary net worth, even the name of your real spouse. As much as you would like to imagine, you have no choice in those matters. But G-d never decrees whether you will go out and search for a spouse or not. That's a mitzvah, and that's up to you. Neither does He decree whether you will use your talents for good purposes or for bad, direct your personality traits in positive or negative channels, squander your money on toys and fancy clothes or to support worthy causes and needy people.
Basically, when G-d declares something to be, it already is. But when G-d asks you to do something, He is thereby handing over to you His unique power to make a choice.
There is another way of distinguishing between the two realms: One is called the outer world and the other is the inner world. The outer world is contingent and pre-determined by the inner world. The inner world is contingent upon the free will of G-d and Man.
The outer world comprises all the natural order -- earth, water, air and fire, all the material and energy on which the world runs. It serves as sort of the backdrop or stage on which the drama of the inner world occurs. It must be the way it is to fulfill the design of the inner world. The design of all the props and sets of the outer world are determined by their role in supporting the choices made by the characters of the inner world. There is no free choice in this outer realm -- just dumb props. Things are the way they are because they have to be that way.
The inner world is comprised of people and of all those things the Torah requests of people -- the activities G-d decided He would like or not like before creating the world. Just as G-d decided on those things He would like and those He would not like entirely out of His absolutely free will, so too we decide whether to fulfill them entirely out of our free will. This is the meaning of the statement of the talmudic sages, All is in the hands of heaven, except for the awe of heaven. The awe of heaven is the key to the inner world that is in our hands.
Does this mean we humans can really mess up the Divine Plan? After all, G-d had an objective in mind when He started this system going  -- meaning a plan for His creation -- that eventually, through our work, He would get the sum total of all He decided He would like: A physical world that expresses His essence.
So, is it possible that we little creatures could abuse our free choice to the point of diverting the world from that ultimate goal?
Absolutely not. Nothing can stand in the way of the Divine objective. After all, He isn't G-d for nothing. At every moment, no matter what occurs, the entire cosmos is accelerating in an upward spiral towards His goal, and there is no creature that can do a thing to steer it in any other direction. We could slow it down, or speed it up. That He left up to us. But we can't turn it around.
How do we know this with such certainty? Simply because to say otherwise would be polytheism, and the Torah absolutely negates polytheism. If there were another being in the universe that could get its way over G-d's -- well, then how many G-ds do you have?
To put it another way: He allows us to go against His inner will, but nothing can go against His intent. And yes, there are things He intends to occur in His world that are against His will. He put things here He really doesnt like. But thats for a whole other essay. Suffice it to say that otherwise it would be a very dull world.
Not only the final goal cannot be obverted. At every point in a person's life, s/he will find him/herself exactly at the spot G-d desires this little creature should be at this time. Make whatever choice you want, it doesn't change the ultimate outcome. It only changes one thing: Who's responsible for the outcome? Is this mess all your fault, that you should have to clean it up? Or are you the hero who contained the damage from extending further? Whose side are you on and how hard are you fighting? When we get there, will it be through your efforts, or despite them? Will we have to drag you there, or will you come marching in the front line? You'll go through the circuit, but will you make the best out of each ride, milking every experience for all its got? Will you overcome the darkness in a fell swoop of the sword, or will you roll with it in the dust, fighting it until its very essence is annihilated?
However it goes, we will get there.
For the words of the Lubavitcher Rebbe on this last topic, see Likutei Sichot, vol. 5, page 65 ff. For greater depth, see Maamarim Mlukatim vol. 5, pg. 154 ff.
On to the next question:
Since there is nothing else but His Oneness, what room is there left for us to make any difference?
The Paradox of Free Choice - Six Questions
1) Determinism: Isnt everything predetermined by the mechanics of the universe?
2) Robotism: G-d knew exactly what I was going to do when He made me this way. Im just a programmed machine. How can I be blamed for being what I am?
3) Prescience: Since G-d knows the future, what choice do we have in it?
4) Omnipotence: G-d wants something to happen and it happens. So how could I possibly choose to do something He doesnt like? Whos more powerful, after all?
5) Oneness: Since there is nothing else but His Oneness, what room is left for us to make any difference?
6) Primal Cause: If G-d is the Primal Cause, doesnt the buck stop there?
 Its important to note the basis of this explanation: That G-ds abhorrence of evil is not necessary. If it were, if you could imagine that G-d just must hate evil, that G-d and evil are opposed in essence, then they would not be able to coexist. But since G-d abhors evil only and simply because He decided to abhor evil, therefore evil can be in His world which is filled with nothing but Him. (back to text)
 Sure, He has no need for the whole thing. Its not like Hes gaining anything out of it. Thats why we cannot say He had a purpose in deciding He would like this and not that. He had no purpose, because He has no needs. But He did set an objective. (back to text)