I have always been a shy guy, been told I was second-rate, etc. I think this is present in my body language throughout the day, wherever I go and whatever I do.
I can walk into a room packed with people, and not get a single look from a girl. It really gets on my nerves, because if girls would actually show some interest in me, I would go talk to them.
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Could be in part looks. Looks are useful for getting initial attention. Great thing about being a guy, though, is that our personality trumps the looks department. A confident personality is the key. So your real question should be to how you can improve your confidence, and as you noted, your body language.
Make a conscious effort to stand up straight, shoulders back. Do all you can NOT to cross your arms. If nothing else, hands in pockets. I like the thumbs-in-back-pockets or index/middle-in-front-pockets (like holstering guns). Until you get more natural with making open gestures, this will keep you from exuding a "closed-off" vibe with crossed arms.
Your last statement says a lot about you, and it's not a good outlook. "...if girls would actually show some interest in me, I would go talk to them." heh, get used to being a guy, it's our prerogative to make the first move. Sucks, I know. But ever think that those girls are saying, "I wish a guy would just come talk to me..", all the while thinking that their "obvious" signals are being ignored? In reality, we're just often ignorant of them...
Anyway YOU need to go talk to them IN SPITE OF any interest or lack thereof. Because THAT will improve your sociability. Pick a person, know some basic things to ask about, and leave within 5 minutes. It's as easy as, "nice chatting. TTYL." If they liked you, they feel good. If not, they're glad you left. Either way, YOU got some conversation practice out of it, and it will make you better. Lastly, analyze things later on, replay the scene in your head and figure out what parts were good and what could've been better. Then visualize for next time. Good luck.1