Prayer text does not always have to be prescribed by someone else. G‑d understands your language too. An appropriate prayer before a major exam would be something along the lines of, "G‑d, please help me pass this exam." Perhaps you could elaborate on that and explain to G‑d why it means so much to you, or why it's important for you to do well. It's always good to get into the habit of praying to G‑d in a manner of talking to Him. You'll begin to feel that you have a working relationship with Him, and you're not just always reading a pre-prepared script at set times.
If you'd feel more comfortable having a standard prayer to go along with that, my suggestion would be Chapter 121 in Psalms. The theme of the psalm is the general assistance G‑d provides us, and our success being dependent on Him. It's short and convenient. I've said it on a number of different occasions when I felt in need of some Divine assistance.
Wishing you lots of luck on your test,
Malkie Janowski for Chabad.org