Use your largest cloves for planting and use the rest in the kitchen. But if all your cloves are small, you'd be better off buying new seed. Success does depend on the size of the cloves.
I dig a trench and work in a lot of composted cow manure. Garlic has a big root system and really appreciates rich soil. Also, in spring I side-dress with 10-10-10 - this year it was Espoma, an organic mix.

I've haven't paid for garlic seed in ten years. I've taken the largest heads of each harvest, about 25, and set those aside for the next fall planting. When I go to plant I break the heads up and plant the largest cloves and I eat any small cloves. I grow hardneck, so there are very few small cloves, maybe one per head.

Another benefit of not having to pay for garlic seed each year is the garlic you grow and replant, will adapt to you're growing environment. It should get better every year.

As far as amendments, I add rabbit manure, composted chicken manure, coffee grounds and some blood meal at planting. In the spring, I side dress with bood meal or other high nitrogen fert and do a soil drench with fish emulsion fert.

Another message I hear constantly is weed, weed, weed! Garlic doesn't like sharing. I actually managed to get almost every weed out of my garlic patch. I was so proud of myself. Now it's coming back and trying to shoulder my carrots and lettuce out of the way, though.

Weeds wern't a problem. I put mulched leaves over the newly planted cloves and in the spring I put newspapers between the rows and the sprouts, then grass clippings over the paper.
The weeds stayed out all spring and summer.

I +mulch with pine shavings - cheap, light and clean. and it sure keeps down weeds.