Looseleaf, butterhead and romaine are the easiest and most nutritious lettuces to grow. What is commonly referred to as iceberg lettuce is generally a waste of time for the backyard gardener, as it is fussy and, frankly, not nearly as tasty.
Germination of lettuce seed requires two elements: lots of moisture and light. Do not bury lettuce seeds, but rather scatter them over some fertile, fluffy, moist soil (you may want to amend with some light fertilizer like earthworm castings before planting), then sift a small amount of compost on top of the seed and press down gently. You will probably see germination in about a week if soil temperatures are around 50 degrees.
Lettuce can and should be grown outdoors in a sunny location as long as temperatures stay above freezing at night - if an unexpected freeze comes, simply toss a few layers of frost cloth on top of the plants and they should be fine.
Make sure the soil stays evenly moist as the plants develop. Thin seedlings to allow about 6" between plants (or plant 6" apart if you aren't starting from seed), but don't worry too much about overcrowding.
Leaf lettuces are particularly fast-growing; they reach maturity in 45-60 days, while romaine and butterhead/bibb types can take up to 70 days. You can harvest individual leaves to eat, or give each plant a crew cut about 2" above the soil line and let it re-grow.
There are many beautiful, tasty types of lettuce in all sorts of shapes and colors. (Check out Lois Tilton's look at some gorgeous varieties in her excellent article here on Dave's Garden.) Some types are more heat-resistant - a definite benefit to those in the sun-baked South - while others are cold-hardy and good for growing in northern climates.
Most varieties of lettuce are generally disease- and insect-resistant. You might see a few slugs in damp weather and aphids when temperatures are warmer. Hand-pick the slugs or use Sluggo, and blast the aphids with water from the hose, or release ladybugs.
The biggest enemy of lettuce is heat, which is why early late winter or spring is the perfect time to start lettuce. Once regular warmth comes along - say 80 degrees or more - lettuce will quickly "bolt," meaning the plant suddenly puts on a growth spurt, then sends up flowers and goes to seed. At that point, it tastes so bitter and chewy that even the bunnies won't touch it.
A note for those with limited space: because it is fairly shallow-rooted veggie, lettuce may be grown in containers (1 gallon or larger). Container growing also carries the advantage of mobility; you can move the containers to a better spot if the weather gets too cold or excessively warm, thus sustaining your homegrown salad even longer.