First, you'll want to visit the gift shop where you'll purchase your tickets for the grounds. While you're there, you will get an "Acoustiguide"—which is something sort of like a tape player with headphones. You can use it to go on your own little self-guided tour.
In addition, there are staff members who give tours every so often. When you check in at the gift shop, the staff will be able to give you a schedule so that you can be sure to get in on these.
The guides take you to places within the house that you cannot go on your own. When I was there in Spring of 2003, our guide took us upstairs and showed us areas that were being restored. It was amazing to see "historic graffiti." Workmen from many years ago had actually signed their names on a wall before putting wall paper over it. It was interesting to think that even then, they realized that they were working for some very important people to American history.
When you come, plan to spend several hours here. The grounds are beautiful, and we really enjoyed taking the time to walk around the grounds and over to the family cemetery and slave cemetery.