You have to pinpoint what you think it may be that turns others off. Is it that you are acting desperate or clingy or that you call them too much and they never call you? Is it something about the way that open up too much to them? Is it the people that you are trying to open up to--they aren't people you would be compatible with?
I'm not trying to blame everything on you, however often times the average person will be turned off by certain things that another person does. First and foremost, you have to learn that you deserve friends, and there are reasons why anyone would like you. You are not unlikable. If you feel that you don't deserve friends or when someone does like you, you wonder why, then that's a problem. You have to know that there is every reason why someone would want to be your friend.
This may take time, damaged people often have trouble doing this because somewhere along the line, someone or something damaged us so that we have these negative feelings about ourselves. it's a step by step process.