It's one of those instruments that takes a few weeks to learn and a lifetime to master. It doesn't take as much breath as you'd think, because the secret is in the technique. Our tuba player is a very small man who jokingly 'hides' behind his tuba, but he plays it very well. In fact, larger instruments can be easier because they have big mouthpieces and you use a 'looser' embouchure to play them.
Cornet and trumpet are not the easiest of instruments for the adult beginner. The mouthpiece is small, requiring a tighter embouchure, especially as you move up to the higher notes (you have to make that 'prprprprpr' thing with your lips whilst pressing them very hard against the mouthpiece). Watch an experienced trumpeter working for a top note and you'll see that even they have to work very hard at it.
Although there are only three valves on most brass instruments, you have to learn how to use them to make every one of twelve notes on the scale at two, sometimes, three pitches. There are alternative fingerings which work better at higher and/or lower pitches, and you would need to learn these too. Then there are harmonics. Depending on how you blow, one fingering pattern can make several different notes and you need to know which is the right one to play. Therefore you need to develop a good ear - that is the ability to tell whether you're hearing the right note or not.
Reading music is something you can learn alongside the instrument. Good sight-reading ability - ie. the ability to play a piece 'straight from the page' without rehearsal - is also a skill that musicians develop over time. Not everyone has it and many get by without it.
If you're really serious, your best bet would be to contact a local brass band and ask if they train beginners. Most do and you're very likely to be able to borrow an instrument from them too. They're very friendly people who are usually more than happy to encourage anyone who'd