This Instructable does not cover the rules of baseball, but the MLB has a fairly comprehensive writeup of the rules, which I won't attempt to duplicate.
Step 1: What you need
First you need a partner.
Second you need a baseball glove and bat. Your bat shouldn't be to heavy or to light also it shouldn't be to long or to short. You should have a bat that is just right.
Also you need a baseball if you don't have a baseball use a tennis ball (not recommended).