Welcome to the United States, and especially, welcome to Disney World!
Depending on the time of year, the weather will be warm, hot, or extremely hot. If you experience rain, don't panic and leave the park immediately; most of the time, showers in Florida pass on through in a relatively short period, and you'll still be able to enjoy your day (likely with a smaller crowd).
Be sure to pack sunscreen, and remember to re-apply it throughout the day. The Florida sun can burn you even if the day is cloudy. Likewise, make sure to drink plenty of water - you can dehydrate quickly, especially in the summertime.
Try to wear light colored, loose fitting clothes that will be comfortable in the heat. Shorts and t-shirts are good unless you are there in the wintertime - and even then, shorts are fine on many days.
Bring comfortable athletic shoes that will not hurt your feet after spending hours walking in them.
Did I miss any basics?