Try to play a few notes. Push down some keys to experiment with different notes, seeing how it changes the strength that you need to use to blow through the horn. try to get a sense of what makes the sound go higher and lower. Just play around for a while.
When you are playing, always cover the holes completely. If you don't, the notes won't come out. Especially when you are using the register key, be sure that all holes are covered completely.