I was in college in the late 90s, and we had a communal shower in my dorm. It was the first time I'd ever been in that kind of situation, and being overweight and having a strictly average penis, I wasn't too happy about it.
But, you know, I got used to it. There were 6 shower heads in a room around the corner from the rest of the bathroom. Just outside of this room was a bench and some hooks.
I'd walk to the shower with a towel wrapped around me, take it off at the hooks, and wander into the shower. Then, I'd try off out at the hooks too.
The showers were normally pretty sparsely populated, and my roommate and I never went at the same time (not that I would care, but I think he did). No one looked at each other or spoke, except the guys that were on sports teams together. I avoided going when the basketball team was in there, because they were all fit and had big dicks (especially the shortest one! A horse-hung, scrawny white kid!).
As someone else said, Always wear flip-flops!
My gym a few years ago had communal showers, too. I was much more confident there, partially because I was older, and I'd lost of a lot of weight. In that case, I just went bare-ass naked from the lockers to the showers. IMHO there's no issue with being not-so-fit at the gym, because everyone's there to improve their bodies, and all should be praised for making the effort.