I know you don't see "communal showers" much anymore but when I was growing up that's all that there was. Yes, we had to shower after sports, baseball in my case, too.
The first time it can be a bit weird seeing your friends naked. Try not to look below the waste. It's OK to get an erection, don't make a big deal of it and hopefully it will go away soon. You won't be alone in the awkwardness.
Remember, erections are NORMAL. If you get one and anyone comments on it just ask them why they were checking you out - throw it back at them - nicely.
It's just a shower. You don't have to make it a long one. Just get in, get wet, and get out. You don't need a 15 minute shower like at home.