Making someone laugh is not rocket science. You can tell a good joke, but that’s not my thing. According to comedians, they themselves have no idea why some joke works and some doesn’t. It’s just a throw them into a wall and see what sticks tactic. The best jokes are mostly improvised, since you if you don’t know what works, go with your guts. Context-appropriate off-hand comment is probably the best for you. I mean, you don’t have to actually be preparing a joke, any off-hand comments could often perk people up. If you can’t be witty, then be sarcastic.
The structure of a joke is the same as the story. An exposition, to a conflict and then a resolution or a punch line. So that could give you a clue on why off-hand comments are often best for the naturally non-humorous. Hell, don’t even put too many thoughts into it. The harder you try, the more you overly think about it, the more you adjust the wording to your naturally non-humorous insights, then it ends up as just a normal sentence. Timing is the essence for this kind of humor, so you want instant comments anyway. Practice if you must, slowly it might come naturally to you. In the meantime, just enjoy the company of your friends and don’t freight!