From a Crown member: First of all, it's great that you are learning another instrument. Make sure when you audition, the staff knows that you haven't been playing long. It's very possible for you to make it. There was a flute player that played mellophone last year, just as an example.
Definitely work a lot with breathing exercises and your basic buzz on the mouthpiece. Then do some basic slurs like Bb-F-Bb, Bb(high) F Bb(low) F Bb(high) D Bb(high). Those some examples slurs used in Crown, but, really most slurs found in Arbans or other method books will do great. Second, I'd work some on articulation since its somewhat different than sax. Work on accents, staccato, legato, and combinations of all three. For Range: as i'm sure you've heard, long tones work great. It will build range and endurance. I can tell you all this, but if you can get a brass teacher to work with you, you will be even better.
Go in with a "kick butt" attitude. Outwork everyone there and show the staff you are serious about wanting to do this. Go to all the camps, it shows you are committed. React positively to criticism. Be open to change. Don't worry so much about the technique and posture; it will be taught to you. You will have a blast.
If you have any other questions, feel free to email me or ask me here.