If you have a few songs that you know so well you can sing them foward and backwards in your sleep, start your set with one of those. Then you can just concentrate on having fun with that song and you'll get past the jitters and the audience will warm up. Also develop the ability to laugh at yourself. Don't worry about blowing a chord or lyric, just smile, laugh, keep strumming and then get back into it. The audience will respond to the fun you're having and that you can goof up and it doesn't bother you. Once you realize you can goof up, laugh at it, and no big hook is going to come out to drag you off stage, you'll find you don't goof up that much.