mrflaytoe,
Preparation for college depends where you are now.
Can you dance? sing? act? How well in each?
Any BASICS you can learn before college would be SMART. I wouldn't say to do too much. Learn what you're already good and focus at that in college.
For example, if you've already experience in acting, don't focus on that until college. They will mold you, if you're willing to be clay. Acting will be the main focus, so learn basics of singing and dance.
Vocal training, tap, ballet, jazz, ballroom, hip-hop, etc. Learn the basics pre-college so you'll be able to get to advanced levels in college.
The more you know pre-college will potentially get you more parts in college, and thus more experience. An advantage over others, if you will.
...
What your asking is a large question. "How do I prepare?"
TONS of advice to give. Simply put: learn your weaknesses, work on them. Learn basics.
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And another thing I wish I did before college:
Make yourself a monologue book, and a song book. Have monologues of any length, and then audition monologues. Have a song book of "performance" songs and "audition" songs.
MONOLOGUES: Include 1 minute, 2 minute, and 3 minute. Include comedia, dramatic, and shakespeare. And for each monologue you include, ESPECIALLY audition monologues, MAKE SURE YOU READ THE ENTIRE PLAY IT IS FROM. In your book, include an "in the moment" summary written by yourself and the title of the play (and include author) so that you remember what the monologue is about. Include who you are talking to and what lead to the moment.
SONGS: Include 16-bar selections for auditions. Include comedic and dramatic. Include classical and contemporary. Include different styles and different songwriters.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle