2 options. The lawn is going to be a real pain in the ass. To get it ready asap you can either cut the sod out or smother it. Cutting it out will remove the grass rhizomes and roots nut removes some good topsoil. To smother look up "lasanga beds". Involves laying down thick cardboard, followed by layers of compost, soil, woodchips, etc. This is a bit tricky to plant lettuce into. It would be easier with larger plants like squash. Hth