Smile at your audience. When it comes time to present, there's nothing that draws your audience into your presentation than a good old fashioned smile. Be happy; you're about to teach your entire class something they didn't know before.
Studies have shown that smiles are infectious[1]; that means that once you smile, it's hard for everyone else not to smile. So if you want your presentation to go off without a hitch, force yourself to smile. That'll make everyone smile; and maybe those smiles will make you actually smile.