Insulting people more often than not creates an uncomfortable atmosphere which is not good for maintaining a conversation.
It's always quite difficult to make friends, it doesn't always work out even for those of us without social issues. But if you're talking about maintaining conversations with friends you already have:
1. Have in mind what topics of conversation you believe they may be interested in.
2. Open by asking them a general question about how things are going and see if the other person zones in on a particular topic. For example, "How are things?", "I had a bad exam today", then the conversation turns into something to do with that exam. If they just say "fine" and give you poor signals with their voice/body language, maybe they're in a bad mood. If they say "fine" and give no such signals, ask them a general question about something you feel they are interested in.
3. In the middle of conversation, listen to what the other person says and either give advice, talk about how you can relate to them or how you're different (preferably the former; doing the latter too much will alienate people), say make an encouraging remark on the topic, or ask a question about what they just said.
4. If they don't appear to know what to say in response, quickly revert back to asking a question about something they said.
5. Make an excuse to leave when the time is right. If you're walking with them, of course the point where your paths diverge is the ideal place to leave - just say "oh, I'm going this way - I'll see you tomorrow". If you're just standing with them, think of something you might have to be doing. Respect the possibility that they might have to do something as well.
It helps to be relaxed for two reasons. The first is that you may remember details from the conversation well enough to keep it going. The second is that you'll probably appear relaxed as well, which is a great asset in keeping a conversation going.
If eye contact is an issue for you, you might have to work on that.
Stories are good, but you should only tell them if the other person enthusiastically invites you to.