What is the term for this speech problem, if there is one?

I have difficulty finding the right words, and putting together sentences in my head, when conversing with somebody. As a result I speak very oddly and can't speak casually and fluently at all, and some people look at me like if I'm speaking an alien tongue or something. And I have to really concentrate so that I don't stutter a whole lot. It's a pretty serious problem, as it's really affected my social life and caused a lot of people to be repelled by the way I act so awkwardly.