Start with low Bb fingering (all keys closed) for all notes. Change the airstream and also "how much mouthpiece you take". The first note is Bb 1, second Bb 2, third F 2, fourth Bb 3, fifth D 3 ...... Do the same with low B and C. If you a re beginner and never done this before you shouldn't do it too much. Just a few minutes every day. You can also try to play a songs in overtones. I use to play "Somwhere Over The Rainbow" too keep up my overtones.
Long tones excercises are also good. Practice with a metronome. c 60 bpm. Try to play with a big volume and also to get every note with a "full timbre". You can add a note bending into this excercise. Try to bend down a quarter step and then. back. You can do long tones in various volume. Start with full volume and the play with less volume in the middle before you take it back to full volume. For me full volume is a controlled full body tone without "overblowing" and at the right pitch. All these excecises can be done in differnt keys and patterns.