How to be more fluent in conversations?

Hi, I'm 18 years old and from DK. I'm quite good at english, not so good at grammar though. What annoys me quite a bit when talking to other people (in English as well as in Danish) is the fact that I usually think for a long time before I come up with a semi-proper response, and I always end up looking dumb or ignorant. I do converse though much easier & faster on a PC.

I have been watching some debates with Christopher Hitchens lately, and the fluency of his speech is astounding to me. He always knows what to say and, when and how. This man is my hero, and I really really want to learn to speak / think like him. Do you think this ability is inborn or something that can be learned through practice and study?

Any comments would be appreciated!!
Thanks.