I'm assuming you're not from North America and that the tests you're referring to are similar to the GRE. If so, there are books dedicated to helping you study for the GRE and, presumably, any other standard test out there. Read through one of those. Discrete math (graph theory, aspects of number theory and abstract algebra), algorithms, computation theory, programming languages, operating systems, and a few other topics will probably be fair game for test questions.

Also, why are you dropping out?