Trying to be funny is the least funny thing you can do. You also don't want to be just funny, if you can take her through a wider range of reactions it will be better. Many guys find it hard because they have to do and say things that they think will drive them away. You'll have to let your unfiltered thoughts out. Some of it will just happen to be funny.
Trying to plan jokes just doesn't work out as well as spontaneously speaking your mind. I do this a lot, but I don't really give a damn and I'm prepared for whatever reaction I get. Sometimes people laugh at things I didn't initially consider funny, because I didn't really think about it before speaking to realize it's funny, but it is.
Being funny is often not so much about what is said, the impact comes from how it's performed. Two different guys can tell the same joke word for word, but one guy can be found funny while the other not. Timing, intonation, facial expressions and body language matters a lot. Not only to great humor, this also applies to seduction or being great in other social interactions.
Being a clown isn't really the best, because people laugh at YOU because you're stupid. That's not the type of funny you should aim for, although a bit of self deprecating humor is good, if you can also convey that you're being ironic.
Being serious is not fun. It deflates any form of tension. It's good to have some tension, it's exciting. It's more fun to try to figure a guy out than it is to get it all laid out on the table right away. Kinda like it's not fun to watch a movie if the ending is spoiled before watching it, you get no satisfaction, tension and excitement if you know how it ends.