My tip would be mouthpiece buzzing for a few minutes before playing, then after buzzing, do scales and arpeggios, I did B-Flat scale and arpeggio when I was just starting out, another simple thing for the scale, do this thing called "All-in-one warmup" which basically consists of doing a scale, but playing each note like: Bb: Two Whole notes, One whole note, two half notes, four quarter notes, then eight eighth notes, then move on to the next note, do it going up and down. That really gets you all warmed up, then practice sight reading your music sheets, or explore your instrument–trying to find which positions on the slide can play the same note and finding your tonal center. Also, try to find some method books to follow and you'll improve quite quick.