OMG Jubilee I want to show too this summer!
I would look for local and open shows.
See if they have a website so you can look at the classes and what's to be expected.
A few shows I was looking at, they have beginner, novice and something else. Beginner is for people who can only w/t and have no or little show experience. Novice if for people who can w/t/c and have little or no show experience.
They also have it broken down into age groups (17 and under and then 18+) I too was worried about doing a w/t class and being with a bunch of little kids.
For clothes, try going to a thrift store, or you can get stuff on e-bay, or you can borrow it too!
I'm not sure what exactly you have to wear, I'm still trying to figure that out (Like i'm not sure if you need a coat, or you can just go in a polo)
Also, see what they accept (tack wise) The shows I'm looking into, they don't allow a few things.