Awwww Jorath, don't put yourself down so badly! It always seems worse for you than for your audience - judging by the spelling abilities of my classmates at Uni for instance (people whom you'd expect to be able to spell correctly), I doubt anyone even noticed your typo. And so many people stammer or falter when giving speeches, it's pretty much expected of you.
Maybe the key does lie in more preparation. You should never memorise exact sentences, but simply be at home in the area which you're going to talk about, so that, even if you can't think of one sentence, you can make up an equivalent one from the top of your head.
It's good to know how to breathe right as Illumina said, but if you have a big prob speaking in front of people, such things might only distract you even further (OMG! I'm not breathing right... hang on, hang on, what was I saying again?). But to breathe through your stomach a few times before you start will calm you down. In fact, if you practise it a lot outside of stressy situations, it will become natural to you.
Anyway, I think as long as you are thoroughly at ease with the subject you will be discussing, you should be safe, since you can rest secure in the knowledge that you KNOW what to say, it's all in your head. I've never had any problems with speaking in front of a class because I came well prepared.