You'll need a packet of lettuce seeds, good dirt and water. Easy right?
1- Find your good dirt spot in a sunny area. Lettuce will grow in the shade too, but a sunny spot will give you a better yield. If you are like the rest of us your dirt is not that good. You'll need to improve it. Some aged cow manure, some peat moss or good old compost will certainly improve your soil. Any gardening center will have all 3 on hand. I had an old compost pile, and that is what I used.
2- Lettuce likes cool weather. Early spring (early as in the ground in no longer frozen) or fall are prime lettuce growing weather. Summer makes lettuce seed and when they seed they taste gross.
3- Make a shallow hole in the dirt, drop a seed in, cover with dirt and water. That is all it takes! keep watering it every other day (unless it rains).
4- Plant about a dozen seeds in the first week. When they sprout, plant another dozen. That way you have continuous lettuce.
Eat them young! don't wait for your lettuce to reach food store size before harvesting, they may never get that big. Tender new leaves are the most flavorful. A good rule of thumb for harvesting lettuce is: cut it anytime after they have 4 leaves.
Enjoy this wonderful vegetable!