I would tell you to play those tunes with the guitarist at least 100 times! Make sure that you know the tunes. Then if you think that stage fright will get you, then find a spot and look there. ie someones shoe, forhead, picture on the wall. Think of playing in the garage where you and guitarist have been playing. But most importantly, HAVE FUN!!! If you should bobble a spot or two, don’t worry about it. Play on through as nothing happened. Remember they want you to succeed! Most likely, none of them can do what you are doing. So I say again HAVE FUN!!
You will be nervous, especially if you are like me. You want to do your very best!
I would not worry about buying a mic just for that, what is provided will do quite well. Now if you have never played mic’ed, then maybe just so you won’t be freeked out! ;D
By no means am I an expert on this, I get real nervous playing in front of 30 at church. Although I will play on the 5th at a prison with a couple of hundred inmates. I say go for it! And make sure you tell us how it went! Good luck and brake a leg! ;D