What part of Texas are you from? I'm about 20 miles east of Dallas and being used to the heat and humidity here on the edge of east Texas, I thought I'd be fine for the Florida weather. However, the heat in Florida seems to be different. It will just drain you! Don't wear jeans down there!! Light capris or shorts with thin cotton T's or tanks is the way to go. I go down there three or four times a year and have been down there the least three years during May. My May 3rd-10th trip will be my fourth May trip in four years. May is a great time to go, crowds are manageable and weather isn't too bad, but it will be hot. I have already tried on alot of my shorts and capris (to make sure they still fit

) . No way would I wear jeans down there. If you, your mom, or your boyfriend's mom could get on ebay, you will also find alot of cute shorts, skorts, and capris for a cheap price. You may also try to borrow some from your friends/family.
Someone mentioned Bodyglide earlier. It's a godsend for chafing as well as preventing blisters. You can also use antipersperant (not deodorant) on your thighs to prevent chafing. Monistat also makes an antichafing gel that works well. You can pick it up at Wal Mart's.
Make sure you carry at least two pairs of very comfortable shoes and switch them out everyday. Personally, I usually wear Teva Mush flip flops or Montrail Molokini flip flops. They're not particularly cheap, but are sooo comfortable.

I can't wear tennis shoes in Florida (too hot and heavy), but alot of people do.
Also be sure to wear LOTS of sunscreen. I picked up some Neutrogena Dry Touch with an spf of 70 the other day at Wal Mart. It's a rub on, but they also have the Neutrogena Spray on Dry Touch with an SPF of 50. The spray came with a little travel size Dry Touch rub on that will be great for the parks. My first trip down there I carried only one bottle of sunscreen, Big Mistake!
Something you need to know is that your hair is going to frizz. I swear mine knows as soon as I step off the plane at MCO. I have yet to find anything to completely stop it. Don't waste your time trying to blowdry and fix your hair every morning. Five minutes after you leave the room, it will just frizz up. If your hair is long enough, be sure to bring some scrunchies or elastic bands and just braid it or pull it into a pony tail everyday.
Mineral makeup lasts longer down there than liquid makeup. The liquid makeup will just melt off of you.
Oh, yeah, one last thing, you will see people in all sizes dressed in all kinds of styles down there. No one really pays attention to what anyone else is wearing (unless it's totally outlandish). Wear whatever is most comfortable for you and just try your best to stay cool.
Good luck, and have a great time!