The simple answer is that you can generally go to any competition which your teacher or your partner is willing to take you to.
If entering with your teacher, your teacher will have to be registered with the organization that runs the competition. Some of the more serious Fred's teachers are, though many of their everday staff probably are not. It's not big deal to do so, but a mainstream competition might be a bit overwhelming for a teacher who hadn't taken students to one before.
If entering with a partner, both you and your partner will need to sign the forms and pay the fees to join the sponsoring organization.
Depending on what you enter, you will have different fee structures and rules ot contend with, and different competition. Of course there are a wide range of divisions at competitions, but attendance in some divisions will be better at some competitions than others.
Keep asking questions, and hope you have fun with it!