I just bought a trumpet to play around with. I played the euphonium in high school band. Any suggestions on mouthpiece size to use, general technique, or just a few tips would be appreciated. Thanks!

A reputable instrument house that handles quality mouthpieces should have someone who can guide you toward the best MP. I like Schilke, but there are many. Try to stay with the "lower" numbers (like 3c maybe) at first. Practice long tones, mid range, and scales and arpeggios, along with lip slurs after warmup. But always stay with the scales and arpeggios, tongued and slurred. It will come.

I started on a 7c mpc...