When I talk to people I sometimes try to be funny and cross the line and say something nasty how can I stop doing this?

Realize what your objective is. Why do you talk to people? I would assume you talk to them to be part of a group. You want to be accepted and liked. If you want to be accepted and liked what type of behaviour will get this tooccur? If you meant someone who wanted to be your friend and they made fun of you and hurt your feelings would you want to spend time with them? I bet you answered no. So, realize that you should talk to people the way you want to be talked to. The jokes are just veiled hostility. You try to play it off as a joke, but it is not really. You are putting someone down to feel better about yourself. Instead be the person you would like to be friends with. A friend that is thoughtful, supportive, shows up when they say they will, empathizes with the rough times you are going through and enjoys doing some of the same things you do.

There is a book out there called, It is so hard to be your friend. It will give you more pointers. Being a good human being takes work every day. Hope this helps.