What: young leaves, shoots, flower buds/stalks
How: leaves & shots raw or boiled; flower buds/stalks cooked
Where: woods, fields, disturbed areas, moist areas
When: spring
Nutritional Value: fiber, some minerals
There are twelve different wild Lactuca species, of which I've only found two to be worth eating. These two are Lactuca serriola (aka prickly lettuce) and Lactuca canadensis. Unfortunately, I mostly find the very bitter Lactuca biennis growing in the Houston area.
Young Lactuca biennis (bitter lettuce).

Mature Lactuca biennis


Both the tasty Lactuca canadenesis and the bitter Lactuca biennis can grow over eight feet tall and L. Biennis can even be double that height in good growing conditions.
How to tell the two apart:
L. canadenesis
Leaves: deeply lobed at base of plant but much more un-lobed, grass-like at top of plant.
Sap: white at first but quickly turns dark yellowish as it dries.
Flowers: yellow.
Height: 5-9 feet
L. biennis
Sap: stays white even after drying.
Leaves: deeply lobed from base of plant all the way to those at top.
Flowers: blue-white.
Height: up to 16 feet.
The young leaves of L. canadenesis have a slight bitterness, even less than some arugulas, and can be added to salads raw. The flower stalks are tender before the flowers open and can be snapped off and cooked similar to asparagus.
L. biennis is extremely bitter even when very young. Boiling in multiple changes of water helps but most people still don't like it. On the plus side, the plants produce a LOT of leaves, so you can get a lot of food from it.