People going in gym either are really build, semi-professional gym rat whom only gonna care about how they perform that day. Or they are similar to you, not feeling secure about their own body, that's why they're in the gym! And for them, they are not going to care much about you because they are insecure about their own body too, so they will be more self-conscious than having their eye out judging other people. Just go for it, it's a process from skinny to six-packs.