I did an Appleseed a couple of years ago. Did day one with my AK and day two with my 10/22. I'd recommend doing the shoot with a .22lr if you have one. Costs less to shoot, easier on the shoulder. There were guys at my class using SKS's and stripper clips but isn't as easy as a rifle with mags. There are some exercises where you'll be instructed to fire ten rounds but in a fire 2, drop mag, insert new mag, fire 8 more. Or fire 5 then five, that kinda thing. Hard to do without mags. You definately need a sling. Canvas military is best but you can use a leather sling or whatever if that's all you have. Sling use is integral to the instruction and once you learn it, you won't want to shoot without one. Bring a shooting mat of some kind. It could be something as simple as a moving blanket, an old sleeping bag, or a piece of carpet from Home Depot, don't need anything fancy. Two things are essential: 1, lots of water. 2, elbow and knee pads. After day one, my elbows were bleeding, ran to WalMart at night, got some soft kids knee pads and pulled them up my arms for elbow pads. Don't forget ear plugs of some kind.
Sights are prefered but they won't turn you away if you have a scope or red-dot. The idea is to make you a beter rifleman so you can teach others, they offer the class free to kids to attract new shooters to the sport. It's a very cool, easy-going, shoot-what-you-brung kinda atmosphere. There's no prize for the latest, fanciest, most expensive AR or anything like that. Some of the best shooters in my lane were using old M1 carbines, Garands, a few AR guys were really good, lots of .22lr shooters were having fun and there was one guy with an SKS with a tech-sight that was just shooting great that day.