I really want to sound smooth and cool but I can't

I really want to sound smooth and cool but I can't I feel like I sound cool in my head, but I just can't in really life. If I make a joke or a funny comment, people usually still laugh, but I'll still feel like I sounded so lame because I stuttered. Sometimes I imagine myself making a joke to someone that I think they will like but I just know that I can't say it without embarrassing myself. I know I can feel comfortable doing this around people I'm close with, but I kind of want to make a funny and witty comment to a stranger at my table in class sometimes.