This is a pretty standard problem for people who aren't blessed with a quick wit. The result for such people is that the stuff that they blurt out is not funny and comes out as trying too hard, or is boring.
Ever had that situation when you are with a bunch of more senior people at your work, and you a bit too eager to please and end up saying something wrong?
The reason that this happens is that you believe yourself to be in a stressful situation. Your pulse rate climbs, maybe your palms get sweaty, you mind races analysing the conversation looking for an set-up for a joke.
Consider another situation. Maybe you are relaxing with your friends, you have had a couple of beers, you are feeling good.
No stress, no worry. You just chilled out cracking jokes and shooting the sh!t.
When are you funnier? When are you more interesting?
The key for you - t'dude - is to turn the "stressful" situations talking with women, into relaxed situations. You need to feel comfortable in doing what you are doing. The only way to do this is to practice. You thinks Tiger Woods would be comfortable on a golf course if he never practised his swing?
Find women you don't care so much about - friends of your sister, ugly women at work/school, women paid to talk to you in shops, and work some short convo's with them. The more frequently you do this, the more relaxed you will be around them.
That's it...there is no magic method...just practice.