I was my brother's best man - and I'm the absolute LAST person you'd think of when you want someone charming and comfortable with public speaking
So basically if I can pull it off, so can you. Just remember that it's a happy occasion, and everyone who will be there is on your side, and will laugh in all the right places and won't make you feel uncomfortable.
I wrote my speech down in the weeks coming up to the wedding, it wasn't too much - 10 minutes maybe, with a couple of jokes and some nice stories. I printed off the script and used it as a prompter during the speech - of course you don't want to read from it word for word.
Look at the web examples though, there are certain things you should say at the end, toasting the bridemades etc, but the structure can be up to you.
Have fun!