Blargh. You're funny, dude, it's not just your friends. It's not something you either "have" or you don't. You can cultivate it and contour your humor to your audience. For example, when I'm talking to my friends we'll talk about the girls we interact with and crack jokes about them and things like that. We also might talk about videogames or other guy oriented things.
This kind of humor won't appeal to girls, so just don't talk about videogames or girls with them. Keep it centered on things that are "universally" funny or funny to chicks.
TV shows like Seinfeld fall into "universally funny." The "Man Show" is guy funny. "Ellen" and "Friends" is more chick funny.
Anyways, the best way to "flow" is to not consciously try to do anything. Just chill out and be a pretty much unreactive zombie. You will find that people accept you *gasp* even if you don't try to regale them with interesting stories or hilarious jokes. They will respect you for not always trying to fill the silence and for being able to relax under social pressure. Once you internalize this, you will naturally be able to relax more around girls and strangers in general, and be more able to think on your feet, which is a pre-requisite to spontaneous humor.
