The first priority is to state your main results and explain why they are interesting (e.g. how they fit in with related work). With only 15 minutes you do not have much time to discuss proofs, but it is nice to give a brief outline of the proof of your main result and what is involved.
As a first-time presenter, I would watch out for the following:
First, as someone commented, it is crucial to not go over time. Don't try to cram too much in.
Second, watch out for the mechanical aspects of the presentation, e.g. is your print large enough to read, and is your voice loud enough to hear.
Third, strive to emphasize the most important points and not get lost in less important details. This is especially important when you only have a short time to talk.
Fourth, know your audience, so that you have some idea what you can assume is known and what you need to review.