OK, it's cliché, but I will write it. Everyone, and I say EVERYONE, is afraid to start a conversation. I consider myself a rather shy person but my voice tends to be very loud and I'm tall so I can't really hide. I've spend years during my adolescence having a bad posture, mostly because I was not confident in myself. I'm not really sure I am now, but I may have some advices:
-Try. Just try. It's difficult, I know, but like when you start learning a new thing, the most difficult part is getting into it. If you want to talk to somebody, then go talk to him for some bullshit reason, it doesn't matter, you just have to break the ice. If that person wants to talk to you, he/she will respond and bring new subjects in the conversation, and it will flow.
-You will feel stupid and uncomfortable, but don't we all? I mean it isn't easy for anyone to start a conversation. And moreover, if you start it, the one you're speaking to will see that you want to interact, and you have the guts to do it. Sometimes it won't work, but hey, that's statistics. No need to be afraid of that.
-Don't trust your eyes. Some of my best friends are people that are the complete opposite of me. Most of our interests are different but that is why we are friends, we learn from each other! So go talk to sports guys or skaters girls, they may looks confident and "scary" but in the end they will be just as awkward as you in the beginning.
-If you're in a party of some sort, drink a bit. Don't hammer yourself, just let the alcohol lift some fears you have. Just know your limits.
-If it doesn't work with a crush of yours well, that's sad, yeah, but like I said, shit happens, leave it behind and look for someone else. You can't be just mumbling about your problems, it won't solve them.
Hope that helps :)