Once upon a time, I wasn't much of a talker either, but somewhere along the line I picked up how to engage in conversation with people. I guess what I do is get the other person/people talking. They usually like it when others take an interest in them or what they do. Once the discussion gets going, you can look for things where you can toss in some comments to the discussion. The hard part is getting a discussion going, but once it does, it becomes easier to continue as you warm up to each other (usually).

As for stumbling over your words, I do it all the time - awkward phrasing, tripping over words, etc. I just roll with it and try to make it a funny situation by saying something like "That was easy for me to say", "Let me re-phrase that in english..." or perhaps slap myself on the cheek and start over.