I have some advice. Listen carefully to your friends when they make jokes. Look out for the recurring themes. There probably will be some. After you've been in their company, scribble some notes down about what sort of things were discussed and what was said that was funny. Analyse what the topics of humour were and try and think of some ways that you could expand or alter the same lines or themes. The idea is to try and develop some in-jokes, because they are always the funniest. They develop over time and take on peculiar significance because they are personalised within the group or within the friendship. You will find it easier, if it doesn't come natually, to go over your notes on what the themes are. Another benefit that comes from writing is that you will be confident that you will always have something funny to say, because usually when you read back over on what was said new things to say will strike you.
Also, try and be spontaneous. Completely relax and just speak your mind. If you are an arts student, or even just a normal, educated person, your verbal ingenuity should enable you to recover from any situation. Whatever you come out with, you can run with it. Try and avoid cliche. When someone asks how you are, don't say 'ok, thanks', tell them something else. Anything that pops into your head, be creative.