I've been doing it for a few years, but I can still not do it. I typically don't get urges. It's just when I'm bored and on the computer, and there's nothing else to do; I find myself doing it.
It's completely normal. You aren't going to grow hair on your palms. You won't develop disproportionate biceps. You won't go blind. You won't go to hell. You'll just enjoy it.
You're fine.
It does not cause blindness, that was a stereotype in the 1960's.
I read an online article and it stated that you should not masturbate when you're sore. So if you're sore, take a break. Who knows what happens if you keep going o_o
The only credible thing I've ever heard against masturbating is that it can potentially cause premature ejaculation when you have sex. This makes sense, as your body is used to getting off quickly... most people don't tug it for 15-20 minutes.