Going to buy a CMP Garand; what else do I need?
Two more so you can use the stacking swivels!
Seriously -- If you are ordering it through the mail, definitely go with the Service Grade. If you get a chance to get to Perry to pick one out, you may find a nice $300 Greek rack grade, but don't count on it. Picking out your own is very cool if you can do it, but I had good luck through the mail.
You will get a military sling with your Garand - learn how to use it. You unhook from the rear sling swivel, push the loop up through the buckle and stick your arm through - pressing your gloved left hand tight against the front sling swivel. Adjust so it snugs in tight against your shoulder. If it is nylon, get a cotton one at a gun show for $10 or less. Even better -- a great first addition to your Garand is a ~$50 Turner leather sling. Here's how to use it:
http://www.turnersling.com/slinguse.htm
"A properly adjusted sling means a steady rifle. "
Get a .30-caliber Bore Snake for range cleaning.
You should also get a good cleaning rod for deep cleaning - others here can probably pipe in with some Brand recommendations - mine has no label on it. Just don't get one of those cheap aluminum, screw-together cleaning rods. Be very careful when cleaning - the easiest way to ruin accuracy is by scratching the muzzle. You can buy a muzzle guard, but I don't know where or how they work.
After sighting it in from a bench, GET THEE AWAY FROM THE BENCH and learn to shoot from position - prone, sitting and off-hand. Start out with a ski glove on your left hand, a heavy sweatshirt, a coat, a pad or carpet for a shooting mat and a pair of binoculars on a backpack for spotting. Move up to a shooting glove, shooting coat, shooting mat and a spotting scope.
You do NOT sling up for shooting off-hand.
Learn to use the windage and elevation knobs. I'll try to find some descriptions on "doping" your rifle - i.e. using the windage and elevation knobs to adjust for...windage and distance.
Aside from that - BA/UU/R!