Cati:
There is an excellent book, called *A Soprano On Her Head*, (which you can get over the internet at Amazon.com) which contains a number of exercises to help with overcoming *performance anxiety*...
Another helpful book is *The Inner Game of Music*....
Books aside, you need to focus on the reasons why you play the trumpet in the first place: the sound of the trumpet, the way it makes you feel, the joy of sharing music with others....
Also, you need to remember that any judge at any competition is also a music educator, or they wouldn't be listening to kids, and that the judge is looking for you to play your very best(and secretly rooting for you to do just that!<gentle smile>)...
As far as freezing on the first note...Since when is music a mattter of *one note at a time*? Music is a process, a *flow*, if you will, not a series of individual notes strung together, but a line of thought and feeling that should flow, not stutter and start like an old car...
Try this analogy: think of the music as already in process before you begin to play...Thus, when you begin to play you're joining with something that is already in process...Sometimes it can help to mentally think through a phrase or two before you begin to play, then play as if you are *continuing* the music, instead of starting from scratch....
I sometimes use the idea of an *eternal airstream* with my students...In other words, picture a wind tunnel that is infinite, which pulls you into the music with the air....
Another idea is that of thinking of a train running by, onto which you hop as you start to play....
It doesn't matter so much what image you use to help yourself through this stage, as it does that you remember that music is really on-going(think of the *music of the spheres*)and that you're not so much starting it anew, as rejoining with it....
Hope these ideas are helpful to you....One final thought is to remember to have fun--If you're enjoying performing, you'll play your very best, and the judge will enjoy it, too! ;^)