Hi there!
By now you may have already done your comp, but in case you haven't, here's the deal.
If you're doing Latin or American Rhythm, *yes* you need to fake tan. The easiest way is to exfoliate thoroughly, then go to a spray tan place like Mystic Tan. If you go two or three times the week immediately prior to the comp (get the medium tan first then the dark tan the last two times) you should be set for the comp.
Your makeup should be extreme. Remember, the lights at a comp can be very strong and without the tanning and the makeup, you'll look washed out. Yup, false eyelashes *are* the way to go.
Your hair should be fixed in a way so that it doesn't fly in your face when you dance. If it's short, spray the dickens out of it so that it wouldn't move in a hurricane. You may want to slick it down first, depending on the style you're aiming for. If it's long, by all means, tie it back, either putting it up in a bun or some other neat way of wearing it. Some people will wear extensions but for your first comp, it'll be too much hassle.
For your dress, double and triple secure it. Be sure you're wearing dance trunks under your dress (they look like briefs and are in a color that matches your dress). If your dress is strapless, (or if it's made with one strap) make sure that you stitch flesh-colored or transparent straps into it (everyone does this) so that there's no chance of it falling down. Practice in your dress, moving up to speed so that you're used to the feel of it and so that you have no surprises from it. (Remember, if your dress can slip or slide from your body at all (especially a latin dress), it can be a problem for you on the dance floor.
Finally, fake nails. Some fun, huh? These should be the last thing you put on, because you won't be able to put your dress on or do anything else after you've done so. Get some duct tape, lay your nails out on the tape the night before and paint them with the nail polish color you want. Then before you dance, slap them on with some nail glue. (This way you won't have to polish them while you're wearing them.)
Good luck!