Solarizing is the easiest way to do this, but it takes a lot of patience. Depending on your area you may need to start now to have your site ready for spring planting. Search for a guide on exactly how to do it, but basically you cover the grass in clear plastic to create a greenhouse effect that will kill the grass and any seeds. You water before putting down the plastic to encourage seeds to germinate. Once you're done (four weeks minimum, more is better) you remove the plastic and plant your plants.

Depending on the type of grass you have, tilling may not be effective. It also may be very difficult if your area is as rocky as you say it is.

Another option is to use a shovel to dig up strips of sod and flip them over. This should be almost as effective as solarizing, but it's much faster (no waiting) and much harder work.