I assume you're a more-or-less beginner dancer as well as beginner performer? And is it a studio showcase?
Your teacher or one of the female teachers in the studio should be able to advise you about what's usual there.
But if I'm right in my assumptions, IME most beginners don't wear full-fledged dance costumes in showcases. There are lots of dance-friendly styles in fashion now and you could likely find something appropriate in a local store. For salsa, the best thing to look for is a skirt that moves and will emphasize your movement, like a flirty skirt, handkerchief hems, etc. Test-dance it both before you buy and before you perform. If there's ANY question about how high the skirt will twirl when you turn, make sure you are wearing dance trunks in black or a color to match your dress (not nude). If you need to secure the dress to prevent "wardrobe malfunctions" on top, body glue, double-sided tape, or toupee tape will do the trick.
A good beginning approach to make-up is to use your usual style of make-up and just put it on quite a bit heavier than usual. False eyelashes are a nice touch if you're comfortable with them. Try it out and stand across the room from a mirror. Can you see your eyes well?
Good luck! Most of all, have fun!