How can I get better at band?

I've been in band since the end of fourth grade and I'm now going into my sophomore year. I started on trumpet then transferred to baritone at the end of my eighth grade year and I absolutely suck! I never seem to know when to come in, like I'll count but it never sounds right when I come in, and I can't play really fast, I was gonna quit band, but I got motivated to do it because I do like band, but I hate it when I can't play crap. But how can I get better, band camp is a little over a month away and I want to really impress my band teacher, considering the fact that I'm not too good. Thanks if you can help!