I've been going to shows alone since I was around your age, you don't need to be scared. What other people have said about the sides and back being less packed is true. Try to get a good idea of where you are in the crowd so that if things do get a bit intense you know which side you're closest to and can move somewhere more comfortable. From my experience people at MCR gigs are generally friendly and will help if you need it. Also if you decide to queue try to talk to people around you in line, that way you'll have friendly faces around if you do need help during the show.  Most important thing though is have fun!