rowuk

That early in the trumpet playing game, I am not sure that sound "quality" is the first priority. While having a good tone is important, getting your breathing and embouchure together will give you the sound as a "by-product". Embouchure is something for a good teacher. Breathing too, but I can give you some tips:
Stand up straight-head straight-neck extended, exhale as completely as possible, then inhale as deeply as you can WITHOUT raising your shoulders. At the peak of inhaling, switch to exhale again WITHOUT holding the air in. Practice that a couple of times until it works. Then replace the second exhale with "play". Try this without tonguing first.
IMPORTANT: at no point should you ever hold the air in, it should ALWAYS be a smooth transition. A good band leader will inhale with the band, giving them enough warning to get a deep breath, in time, before playing.

__________________
Whenever I feel blue, I start breathing again.