Check what you already have
Identify the instrument and mouthpiece you own or rent. If you are unsure about its condition, describe the issue rather than forcing a stiff part to move. A lesson is not a substitute for repair work. If you have not acquired an instrument, ask what to look for before committing to a purchase.
Bring real musical material
A school method book, ensemble part, audition requirements or a tune you want to learn gives the lesson useful context. A beginner with no music yet can still inquire. There is no need to learn a solo just to demonstrate that you deserve a first lesson.
Describe the starting point
Tell the teacher how long you have played, whether you read music and what feels easy or confusing. If you have a performance coming up, give the date and actual requirements. Honest information makes it easier to choose an achievable first task.
Prepare a safe playing space
Leave enough room for the slide, instrument case and a music stand if available. Keep the instrument away from a busy walkway. Choose a lesson time when playing works for the household. For online lessons, a stable camera view of posture and the slide is more useful than a close-up of only the face.
Expect a focused musical starting point
The lesson may begin with sound, a few notes, rhythm or a passage from your music. The teacher needs to hear how the instrument responds and how you approach the task. Rest, listening and discussion can be part of the session; it is not necessary to play continuously.
Know what comes next
Ask which task to practice, what to listen for and how to approach a problem that returns at home. Keep the instructions brief enough to use. For a child, a parent can help record the plan without needing to explain brass technique themselves.