green manure is a term used for a crop that is planted to enrich the soil, it is tilled back into the soil and not harvested. typically it is a annual rye grass or buckwheat. you till it befor it goes to seed, and you get the benefits of added organic material in your soil. this is an old and well tested technique for long term improvement of your garden soil, and most people who practice this technique do it every winter and till the cover crop into the soil in the spring, the rye grasses also will add nitrogen to your soil.
you can also do as many have described till additional organic materieals (leaves, grass clippings, wood chips, vegetable scraps, NOT ANIMAL SCRAPS) into your garden and cover and leave this over the winter to decompose. this may be described as sheet composting if the material is left on the surface and then tilled into the soil in the spring before planting. if you choose to add chicken manure, be sure to add it in the fall, it has too much nitrogen and will burn your plants if added fresh in the spring.
one other way of preparing the soil is to spray the area with roundup, this will kill the grass, till and cover over the winter, and plant in the spring.
good luck, you will love your garden, i love mine,
alex