First you have to know what kind you want. Each group has its own growth pattern and taste characteristics.
Crisphead lettuce is probably the most familiar.
It has a tight, firm head of crisp, light-green leaves. In general, it's intolerant of hot weather and bolts in the summer. Bolting is when the plant sends up a flower stalk to reproduce. Early bolting and a long growing period makes it hardest of the lettuces to grow at home. The crispheads do not grow well in Georgia.