Well, i started with a 7c because it came bundled with my trumpet. Depends on your "blow" power (nothing obscene between the commas, please!). See if you can make use of your currently mouthpiece. My teacher recommended a smaller size (2c) but i`m okay with the one above.
Mouthpiece advice: Ask your private teacher. If you don't have one, get one. You need someone who can examine your physical characteristics. The same mouthpiece for everyone is not the best way to go.
You may end-up with the same mouthpiece as almost everyone in your class, and that's ok, but mouthpieces are like underwear. Too big and "baggy" will be uncomfortable. To small and "tight" will also be uncomfortable. Both will make playing harder than it needs to be. Trumpet playing is difficult enough without adding obstacles.
So if there is no qualified teacher available to help (please DON'T ask the "big kids," they usually don't know either!), here's a starting point: It's all about "response" and "tone."
1. Many beginners use the equivalent of a Bach 7C, 5C or 5B. There are some professional who use mouthpieces of the same size. If you have average size lips and teeth, those will be ok, even if not the best option. Remember that with Bach, the rim size and contour may change without warning. EX: The 5C has a slightly shallower cup than the 5B, but it also has a sharper inner rim. The numbers don't tell you that. You'll need to go the the Bach website to see exactly how each model is constructed.
2. Starting with a Bach 7C, 5C or 5B, which mouthpiece is easiest to buzz? If it buzzes easily, you are a least in the ballpark as far as cup size, diameter and rim contour and size.
3. Experiment with other mouthpieces in that range. Try a Bach 3C. It's not much bigger than a 5, but is more comfortable and may respond better.
4. When you find 2 or 3 mouthpieces that buzz easily with a "big" buzz, then try them on the horn. Sometimes the one that buzzes most easily is the right mouthpiece. But sometimes the one that's takes a little more work (AIR) will get a better sound.
While anything from the 3C to 7C will probably be fine for the first year, as you grow, you will probably need to change mouthpieces. Best advice is still this: get a good private teacher!
let me put it this way... despite what alot of people say, go to your local sam ash,shop,etc. let the mouthpiece choose you. try alot of them. personally i started on a bach 3c. now im on a denis wic 3c. hope this helped and good luck!!