Take your voice part into consideration. If your most comfortable range is Alto and your Soprano range is not as strong, do not audition for a Soprano role unless you are really able to get there through a lot of practice. The same goes for every other voice part. If you do not know your voice part, go see Mr. Machan in the choir room and he will help sort you out.
Take your acting strengths into account as well. If you play childish roles very well, perhaps consider auditioning for a childish character. If this is your first acting role, consider your personality and whether or not you would like to play a character that is similar to yourself. With that being said, it is perfectly acceptable to challenge yourself and play a role that brings about opportunities to grow as an actor. Do whatever makes you feel most comfortable!
Consider your interests as well. If you just love a character and they fit your voice part well enough, go for it!
At the end of the day, it all comes down to what you want to do and whether you believe you are capable.