I'll be presenting at my first conference in 2 days, and I'm not really sure what I'm supposed to be doing, to be blunt. I was the second author in a paper that recently got published. So I more or less have an idea of what I need to discuss. I'm limited to at most 15 slides but what I'm dreading the most is the question and answer section. Can anyone tell me how these presentations are usually done and/or give hints? (I'm a grad student in math, by the way)
I'm not sure I really understand what you want to know. Have you never seen a conference talk? (If you haven't, you can watch some here.) Perhaps you're going to different conferences from me. The question and answer part is usually 2 minutes at the end of the talk; occasionally no one has a question, but often to be polite people will ask for a clarification of some point or ask about a connection to something they free-associated about during your talk. You should do your best to answer, but people will understand if you say "Gee, I hadn't thought about it that way" or "Let's discuss that during the break." Generally people are eager to get to lunch/coffee break and won't draw it out too long.
Ask your advisor for a chance to practice by giving your talk in front of your own research group. Or arrange to give it in front of other grad students, possibly through your local student chapter of SIAM. Make sure your "practice" audience knows that you expect them to ask questions.
The length limit is given in terms of time, not slides. Practice your talk and make sure to finish on time. And follow the advice given here: http://math.ucr.edu/home/baez/advice.html.
When the questions come, relax. You can't go wrong as long as you don't try to talk about things you don't know. It's perfectly acceptable to say you don't know. If there are no questions, it usually means nobody understood anything useful from your talk.
Giving good talks is a vital career skill - yes, even in math! At the conference, pay attention to talks that you enjoy and how they are delivered. Improving your presentation skills will be a lifelong, valuable effort.