How to Become Sociable

Steps
Look at other people. They're like you - the only difference is that they believe in themselves. A sliver of self-confidence goes a long way, so try to believe in yourself.
There are people around you who are funny and sociable. Analyze their behavior and see what they do. Identify their cues and attempt to assimilate some of their beneficial patterns into your demeanor (see Warning below).
Talk to others around you. Don't let what others say about you pull you down; just talk it out. Doing this will help you vent to people, which will make you feel more social. Ask questions, tell jokes, do whatever it takes to start a conversation.
Say yes. Always. Someone says a group of people are going to get pizza and they want to know if you want to come. You have to meet the FedEx guy because the package you're getting needs a signature? Not anymore. Now, you're going to go get pizza with your friends. There is no reason you should ever need to say no to a social obligation except if it interferes with a pre-existing one. Other than that, you're free Wednesday for pizza... And Thursday... And Friday, too... (So what if you're tired after a long week? Sleeping is for Saturday mornings.)
Ask your friends to talk about you, so you can find out how your behavior is perceived. Then, ask them how you can improve.
When you are at home, call your friends and talk to them on the phone for practice.
Always try to accept others' offers and invitations and be with them. Also make invitations to others; it's a good way to show people you enjoy them and they'll like you for it.

If someone cracks a joke and everyone falls about laughing, and you haven't been involved much into the conversation, join in laughing or agree with what they're saying (only if you actually agree though!). People will appreciate you and involve you more.
Go up to someone and just start a conversation with them. This will let them know that you want to talk to them and they will want to talk to you.
Don't be by yourself all the time! If you don't know a lot of people you're around, just walk over and start talking. Most likely, they will talk back to you.
Sit with a whole group of people at lunch, not just one or two friends. You can be more sociable this way, and around a lot more people.

Tips

* Always believe in yourself.
* Believe that you are exactly who you want to be.
* Don't act stuck-up. There is a difference between confidence and snobbery.
* Remember, being social does not mean being everyone's friend. It is okay to be selective with friends and a little cautious. It's safer that way and ensures you get quality friends, not quantity. The main test of if you should be friends is this: In general, do I feel good after being with this person? If not, your intuition is telling you something and it is time to move on.
* Compliment other people. What you compliment them on may lead to a conversation.
* Don't overanalyze what to say before saying it.
* Go with the flow.

Warnings

* Don't mimic your friends exactly. Just identify the things you find helpful, and form them into your social routine. However, do this without losing your respect for them. If you become cocky about it, over-arrogance will somehow get mixed into what you assimilate from them, and that won't be very social. Funny how that works. So respect them. They are just as good as you, in case you start to think you are better than them. This will keep your social life strong and productive.
* Don't talk all the time just to not seem shy. This can get very annoying. Listen, then add to what the other person said.
* Don't talk badly about others. This will not win you friends.
* Don't be afraid to act silly or funny, but know where to stop.