1. DO wear a dress or a skirt shorter than the length of your robe. DON'T wear pants. The gown looks best without pants below it. Also, because the part of the dress showing below the robe never matches the color of the robe, wear a short dress/skirt, not maxi.
2. DO wear whatever shoes rock your world. Your shoes/sandals are your only signature item--the rest of you is basically in a uniform. So wear whatever shoes/sandals you want (of course, I'd typically go with black with about a 2-inch heel, but you get to do whatever you want here). You can make a statement with your shoes/sandals if you want to (ditto, your earrings).
3. DON'T wear jeans and tennis shoes under your gown. Really.
4. DO wear something lightweight under your gown; you'll be hot no matter what you wear. If I were you, I'd wear a tank or cami under your blouse/dress--it'll keep you cooler and keep your clothes nicer (i.e. no sweat stains) for after the ceremony.
5. DO steam your gown before graduation if it has big fold marks in it. I was at a graduation last weekend & saw most students walked across the stage & took photos with the fold marks--not really the ideal look.
Mortar board tip: DO wear your mortar board flat on your head like it's supposed to look. DON'T tilt it back hoping it'll make your face look cuter--you'll just look silly.
One last tip: If your graduation is outside, you & everyone attending will be melting in the heat. I was once at a graduation where they'd set up a table with water bottles for one specific group. By about halfway through the ceremony, people were literally fighting for those bottles. Parents were heard citing how much they'd spent on the education at the school--the least they could expect was a bottle of water! (They said :) . Anyway, do yourself a favor & bring along a water bottle & tell your family to do that same. You can leave it at your seat when you cross the stage.