They open up the lots 4 hours ahead of gametime. You can tailgate in the parking lots or you can cart your stuff over to the great lawn. If you want to tailgate on the lawn, I suggest you secure a West or West Preferred Parking Pass. Otherwise, you are in for a hike. There is usually some entertainment on the lawn, and it is cooler to party on the grass than the asphalt.
Nearly everyone brings their own pop-up canopies, which are supposed to be no larger than a 10x10. Security does not allow charcoal grills, but I have seen them used.
If you are coming in multiple cars, make sure you arrive together. Security directs you to your parking space. (I ignore them, but spaces around you will be filled up quickly, so you can't save them).
Cook whatever you want (burgers and brats are always a good standby). I would suggest bringing lawn chairs as well. You may also want to bring something to tailgate after the game. The lots are terrible to get out of, so I usually just tailgate for another hour to let traffic die down.
Hope that helps.
Go Cards!