When I was your age my then boyfriend brought me to Disney World to celebrate my graduation! It was such a magical trip that years later I married that boyfriend! See, you'll know he's the one if he asks you to go with him to Disney.
I suggest that you look at one of the
Magic Your Way packages that Disney offers. These can include your resort, park tickets & even dining all in one package which will help take a lot of the guess work out of your planning.
Definitely visit The Magic Kingdom for that true Disney feeling. Epcot is a lot of fun & if you go to the World Showcase there you can meet lots of Cast Members from all over the world that are around your age. My husband & I are still friends with people that we met back then in Epcot! Plus, in Epcot's World Showcase you can experience different cultures & their foods. Downtown Disney is free to visit & has lots of neat shops & restaurants.
Pack lots of layers! In the early morning or evenings a hoodie or cardigan will be your bff! You'll be doing a lot of walking so be sure to bring shoes that are comfortable & a backup pair in case you encounter rain.
I know this is a lot of information so if you want us to narrow some of this down for you just drop another question at us with specifics! I hope you have a great time, Brittany, & congrats on graduating!