A smile helps a lot. But it have to be real. Look forward to talking to people. See it as a challenge! If you're walking past someone and get eye contact, a simple "Hi!" and a smile is great.
Try not to take it serious. You come across a lot better if you are yourself and relaxed. So what's the worst thing that could happend? You get a light conversation that doesn't lead anywhere. That's still good! Every bit of practice will make you better
Always try to look for an opportunity to throw in a joke, about anything, when you're talking to someone or trying to approach someone. It doesn't have to be that good. I find that it helps a lot to break the ice, and the conversation doesn't have to be all serious and awkward.
Lots of times people just aren't interested in a conversation, and I find it pretty easy to spot. I'll just back off. Boring people! ^^
Can't think of anything more to say. Take my advice with a grain of salt. I'm certainly no expert, and I don't have a girlfriend, hehe. But be natural, have fun and keep that smile up. The more you try it the easier it gets
Good luck