Get a bowl, and a head of lettuce. Make sure you wash the lettuce well. Break the leaves off, and rub the lettuce under cold water. Then dry. To save time, simply buy a bag of pre-washed and shredded lettuce. If you would like to, you can buy mixed greens or a more "exotic" bag of salad.
Romaine, Boston, and red-tip lettuce are all great options for a salad, as are radicchio, spinach, curly endive, and kale.