If your oven is of the self-cleaning variety, koshering it is fairly simple. All you need to do is self-clean it. If you need to kosher the oven racks too, just leave them in the oven while it self-cleans.
A non-self-cleaning oven is much harder to kosher, requiring a lengthy and thorough procedure that involves a blow torch. If your oven is of this variety, I would recommend that you contact a rabbi who is familiar with both the laws and the procedure, and can arrange someone to kosher it for you.
Visit our Kosher Kitchen section for more on koshering various kitchen utensils and appliances.
Rabbi Eliezer Posner