At a minimum, all you need are a quantity of 8 round clips equal to the number of rapid fire stages you will shoot and EITHER one 2 round clip, or an extra empty 8 round clip for loading two rounds with.
The trick for using an 8 round clip for loading two is to cross the cartridges so they bear against each other and the sides of the clip. Shove the clip into the rifle and the rounds will straighten out. Push down slightly on the cartridges with your thumb while closing the bolt over them. That will lock the partially full clip in the magazine. Then, to chamber a round, simply pull the bolt back and let it fly. With the Garand, unlike all other rifles used in NRA Highpower, you are authorized to close the bolt while standing waiting for the targets to come up for rapid fire.
The reason you only need one clip for 2 round loading is that it is very easy to refill a clip with two rounds between rapid fire strings. Refilling 8 rounders is a PITA when your are hurried.
As far as ammo belts, don't worry about it. You are not required to load from a belt pouch, like some of the military matches require you to. Any old range bag will suffice. You take the 2 round clip with you when you stand in preparation for rapid fire, and leave the 8 round clip on the mat somewhere where you can grab it for the mid-string reload.
Most beginners get a medium sized range bag, tool bag, gym bag, or similar to carry their gear around.
The slow fire stages are shot with only one round at a time in the rifle, so no clip is needed. A SLED is nice to have, but not necessary. The Army's FM 23-5 describes how to load one round at a time safely. Basically you insert the round in the chamber, and while holding the oprod handle push the follower down enough to unlock the bolt. Slide the bolt down to about halfway closed, then let it fly so that it locks up. That method does not generate enough momentum to create a slam fire. And that's how I used to load until I bought a SLED. Buy the SLED eventually, but don't let the lack of one stop you from competing.
As for the rest of the gear, here's the bare minimum:
A spotting scope with a stand or a pair of binoculars to see the shot spotters on target and to read the wind mirage.
Something to lie on. A piece of carpet long and wide enough to cover you is great. A purspose built shooting mat with a waterproof bottom is better.
A heavy glove for your left hand.
A sling. If you don't have a competition-grade M1907 leather sling, use the M1 cotton sling. Competition M1907s are very heavy duty, and the cheap GI replicas will not take the heavy use given in a match. You are much better off with a $5 cottong M1 GI sling. Have someone at the match show you how to rig it correctly. You have NO hope of shooting anything close to a good score without a sling in sitting and prone. The sling has to be on the rifle, but can't be used for support when standing.
A heavy jacket or coat, and a sweatshirt to wear under it. This will serve several purposes. While you won't get the support in standing that a true shooting coat will give, it will provide cushioning of your shoulder and bicep when shooting with a sling, and will give your elbows some protection and slip resistance when shooting prone.
Other than that, go and enjoy.