This is why you should not take something for the first time right before doing something important to you. If you had tested it out first, you wouldn’t have needed to ask this question. Also, drugs like Concerta are not “when you need them” pills. If you don’t take them in a regulated way, they can seriously alter the biochemical reactions in your brain.
The possible side-effects mentioned by @Tropical_Willie might not come about in your case, but at this point you don’t know. Go back to your doctor, say that you want to do something about (or take something for) your ADHD, and go from there. Like @linguaphile said: you need someone who knows you’re taking this to watch for bad side-effects.
As for test-taking tips, it’s a bit late for those now. I generally recommend using mnemonic devices and not studying the night before an exam. This latter bit of advice requires one to schedule study time in a way that typically prevents the unfortunate practice of “cramming” for the test. It also makes you more likely to guess correctly when you cannot consciously remember an answer.
I wish you luck on this exam. Maybe you’ll find you don’t even need the pills. But if the worst happens, you know some things to do now. I hope you do well, but it’s more important to be well. Good luck!