Well don't panic! Help is at hand! I used to be just like you but now I go to parties all the time (two in 2012. Mental!) and I'm here to pass on all the tips I know on how to make you the social butterfly you long to be. Buckle in and prepare to be popular.
First of all TARGET YOUR DESIRED FRIEND. You can't start off being greedy; you have to be specific. Pick a person you want to be your chum and work on wearing them down. You can achieve this by doing the following:
i. Like every status update they post on Facebook.
ii. Like all of their holiday photos on Facebook.
iii. Poke them all the time on Facebook.
After that you come to stage two: INVOLVE YOURSELF IN EVERYTHING. People won't just welcome you into their lives unless you force your way independently. Turn up uninvited to their parties, arrive at every location they check in at on Foursquare*, buy all the clothes they buy. This will prove to them that you are interesting and cool.
*You should really consider whether or not you want to be friends with someone who still uses Foursquare.
The next step is one of the most important of all, it involves CONVINCING ALL OF THEIR EXISTING FRIENDS THAT THEY'RE HATED. If your target already has other companions why would they ever consider choosing you instead? Get to know their other friends and spread seeds of doubt about their personal worth. Gossip openly about how your target said nasty things about them and, if you must, frame them for murder. It's a lot of work but it will all be worth it when you're sipping a cool beer with your new friend on a crowd free Friday night.
Finally, you must absolutely NEVER ACCEPT DEFEAT. There will be times when your targeted friend insists that you, 'leave me the hell alone!' and, 'stop following me around, you freak!' but you can't let that reception get you down. If you give up at the first hurdle then you'll never have any friends at all and you'll be back at square one. There's only so much help I can give you, you know?