I don't understand how to have a regular conversation, what am I supposed to talk about? I can only hold a conversation for a few minutes at a time with close friends and almost not at all with anyone else. Please help.
What you don't understand is that 90% of all casual conversations are about nothing at all - or, rather, whatever they're about is not their main point.
Follow this digression: monkeys and chimps like to sit together and pick nits off each other's fur. Now, getting rid of nits is good, but money followers have discovered that it's really just an excuse to sit together and enjoy being friendly.
People generally don't pick nits off each other. What we do is sit around talking about nothing at all - the weather, sports, last night's TV, girls, whatever. But sociologists have shown that a half-hour later very few can remember what the conversation was about, because it was just an excuse to hang out together.
Don't worry about being interesting, just be part of the bonding. An occasional "Yeah" or "That's right" will hold up your part of the conversation, and just about anything you add to it will be accepted because WHAT anyone says is less important than THAT they're being part of the fur-cleaning group.
Sounds to me like you don't have much to talk about, or interesting people to talk with. Try taking up a wholesome hobby like carpentry - soon you'll be nattering away all afternoon with woodworking friends, discussing the latest lumber, tools, and techniques you've come across!