How to cook: Chicken for Mexican?

[Jimmy W]

Say, I want to make Mexican food tomorrow. Hard or soft tacos. And I want to substitute Ground Beef with Chicken. So either strips or cubes of chicken.

1: What do I buy when I go to the store? What's the exact name of the thing I need to buy? Is it something I order at the counter, or is it in the cold area with the Ground Beef?
2: How do I cook it?
3: Do I cut it up and then cook it? Or cook it, and then cut it up?
4: Do they sell pre-cooked diced or strips of chicken?

(I tried to cook chicken once, and it went horribly, horribly wrong. Ironic, since I deep-fried thousands of chicken when I worked at Wendy's.)