Hi Jennifer!
Some panelists wrote about planning a first trip
here. I also recommend ordering the free Walt Disney World planning DVD, which can be done
online. I also highly recommend buying the book "The Complete Walt Disney World" by Julie and Mike Neal. It is an excellent book for first time visitors to Walt Disney World and it is full of beautiful photographs to give you some idea of what is offered at Walt Disney World.
In terms of crowds, there isn't much difference between the end of September and early October. I personally love the month of October as Mickey's Not So Scary Halloween Party and the Epcot International Food and Wine Festival are fabulous events that shouldn't be missed. Also, there's the potential for slightly cooler weather, but again there won't be that much difference between the two weeks.
After you decide when to go, the next step will be deciding where to stay. My favorite Disney Value Resort is Pop Century, my favorite Moderate Resort is Port Orleans Riverside, and my favorite Deluxe Resort is the Wilderness Lodge. As a first timer, I want to remind you to make Advanced Dining Reservations (ADRs). ADRs can be made up to 180 days in advance at (407) WDW-DINE or online. Many of the most popular restaurants book up early. I suggest considering some Character Dining Experiences; my children absolutely love Chef Mickey's in the Contemporary Resort.
Hopefully this will give you some ideas on how to get started. Write back if you have any more questions!