Hi Nichelle!
First, I recommend ordering the free Walt Disney World planning DVD. It will give you a great overview of the Walt Disney World Resort and the various hotel options. You can order it
here.
Second, I recommend picking up the book "The Complete Walt Disney World" by Julie and Mike Neal. It's a great book for first time visitors with just the right amount of information without being overwhelming. It's also a beautiful book full of color photographs It will give you more of an idea of everything there is to do at Walt Disney World and give you tips to plan your trip. If you want even more information after reading that book, then I recommend getting "The Unofficial Guide to Walt Disney World" co-authored by Len Testa.
The weather will be much nicer in late March / early April, but the crowds will also be greater during that entire time frame. For the second set of dates, the crowds will be very high, but actually less than the Spring time with the exception of July 3rd and July 4th. Those two days in July will match the crowds in the Spring (maybe even exceed them on the 4th of July). That said, it's a hard decision. Florida is generally very hot and humid in the summer time!
Enjoy planning your first Walt Disney World Resort vacation and write back if you have any more questions!