I am a senior in high school and auditioning for college music scholarships. As one of my songs, I plan to play Stamitz's Concerto in G. I take private lessons, and my teacher is having me write my own cadenza. However, I have never done anything like this before, and I'm not really sure where to even begin. Any advice would be greatly appreciated!

you should talk to your private teacher more about that and get him/her to play one for you. cadenzas are something that are hard to explain and would be better to listen to one to get a full understanding of it. the more you listen to the easyer they are to write.
basicly all they are, are places in music where time stops and the soloist plays by themselfs whatever they've written for however long of time untill the rest of the band comes back in and continues the song. they usualy end in a framata.
they do not have to be long atall but they do need to fit in witht the rest of the song
here is an example. this one is abit too much. but you'll get the idia if you listen the the begining and the end of the cadneza
http://www.youtube.com/watch?v=NzwEij2Rzcs
http://www.youtube.com/watch?v=NzwEij2Rzcs