1. Notice the things that make you laugh or you find witty in conversations. Keep a journal if necessary.
2. Steal liberally.* If the comments are highly situational, think of ways to generalize, or imagine similar situations where you could use similar material.*
3. Focus on people's reactions when you tell those stolen jokes.
4. Once you can get a reaction with tested material, branch out to variations and introduce your own material. (Don't stick with joke theft forever, just use it as a transition, so you can isolate other things to work on.)
* (2) probably requires multiple distinct social groups. Even in the same group, you can get away with this with a little hat tip, like, "It's like that time when John said X..." Or if you build from that into storytelling--everyone likes hearing stories about times they said or did interesting things--so you'll have a sympathetic audience while you practice some timing and rhythm.