Some things to remember
-Make sure you are sitting up with good posture to allow better breathing
-Tightening and loosening your embouchure (mouth shape) will make the pitch of your note higher and lower
-Different combinations of keys play different notes
-Be sure there is nothing in your mouth before you play such as gum and food.
The steps to get you started
1. Shape your mouth in a small o then squish your lips tighter together.
2. Keep the outsides of your o tight but leave the o relaxed enough to make a buzz.
3. Buzz your lips into the mouthpiece while blowing air until you can consistently make a sound with it.
4. Put the mouthpiece into the instrument, take a full breath, and do the same as you did with just the mouthpiece but control your air enough to make an even tone.
5. Try out some different fingerings and work on learning some scales and some songs. Follow along with lessons in a music book for beginners and maybe take some classes and private lessons.
A good book to learn from is the standard of excellence book one for euphonium. You can find it on Wert Music for 10.50$ (https://www.wertmusic.com/products/standard-of-excellence-baritone-bc-euphonium-book-1)