lettuce:

My least favorite commonly eaten food is probably lettuce, but it's good to know how to prep it for a salad or even a sandwich.  There are four types of lettuce:  butterhead, crisphead, looseleaf, and Romaine.  Icebergs, a type of crisphead, are very popular but the least nutritious.  Romaine is typically used for Caesar salads and is also crisp, but not as "sweet" as iceberg.  Looseleaf is common to gourmet sandwiches and green salads, and I find it is the slowest to wilt.

For heads of lettuce, peel off the outermost leaves and remove any part that is brown or wilted.  For shredded lettuce, rinse the outside, cut the head in half, and then slice down very thinly to achieve the strands you find in tacos, for example.  For a salad, whatever type of lettuce, I peel off all the leaves carefully, rinse them, tear the leaves apart by hand into smaller pieces, and place them in a salad spinner (sometimes) to keep them dry and crisp.  For looseleaf lettuce, just cut off the joining stem and proceed as above.

There are a number of lettuce variants like arugula, raddichio, and endive, and each has its own type of flavor.  It's probably a good idea to taste them individually and pair them with various foods to see what you like.

With darker greens, you might see some sliminess that needs to be removed as well.  Hopefully you can eat the leaves soon after purchasing them so as not to reach this stage.