Try a loose fitting white shirt and some kind of long dark jacket left open.
It'll be a little warm and uncomfortable on the arms but it's a lot more slimming than spandex pants and a snake skin t-shirt.
If you sing, you might consider getting a headset mic with a headband.
Once you get under hot lights, you'll be glad you have something to stop the sweat from getting in your eyes.
And if you haven't perfected your 'guitar face' yet, or aren't comfortable in front of an audience, get some sunglasses to hide your eyes.