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).
Step 2: Learning to throw
First stretch a little bit. Then try to throw the baseball to your partner try not to throw it under hand or side arm. Underhand is not good for the arm, and it's better to learn overhand when you're just starting. For now try to stay at least 10-15ft apart. Just keep throwing the ball back to each other for a little while. Try to follow through and at the end of the throw your arm should be pointing at the person you are throwing the baseball to. Practice throwing like picture 3 and 4 with only your wrist now once you have done that for a while practice with your partner. Now put all the pictures together arm up wrist cocked back and end up pointing at your partner.Now you know the basics of throwing.
Step 3: Catching
When a ground ball comes to you bend your knees bend your back and get your glove down with your bare hand on top of the glove and watch the ball in to your glove. If a slow ground ball comes to you run towards and pick the ball up.
Catching pop flys is more difficult. First try to find the ball in the air. Next get under the ball and get ready to catch with two hands. Try not to be too hard on yourself if you don't catch it the first time, and keep trying.
Step 4: Outfield
Let's start with pop-ups/fly balls. You first want to step back in the direction the ball is going. This helps to get a jump on the ball. Then, judge where the ball will land. Simply put up your glove, and with TWO HANDS, catch the ball. Next is throwing. After a pop-up, ground ball, or line drive is fielded, start to run towards your target. Then, jump up slightly and bring your hand back as far as possible. Then, with all your power, throw the ball to your target WHILE STILL RUNNING AT HIM!!! This takes practice. This is known as a Crow-Hop(don't ask). Oh, and always call, "I got it!" to let the other outfielders know that it's your ball.
But the main goal is to not let anything by you. More games have been lost by that than any other mistake. Always go for the ball.
Step 5: Hitting
First throw it up and hit the ball. This is good practice but as you progress it will be better if your partner throws it to you instead so you get used to it. You can use a tennis ball if you don't have a baseball. Another alternative is to go to the local batting cages.
When you are hitting the ball put most of your weight on your back foot then take a little step forward when you are about to hit the ball. You should try to swing level, and make sure your arms are fully extended. Try not to pull your head out while hitting the ball by watching the ball all the way until you see it make contact with the bat, or if the ball goes by your bat. This picture shows how level your swing should be and how he kept his head in not pulling it out. Make sure you choose a batting stance that fits you. I have a batting stance with just my front toe touching the ground and holding my bat about level with my chest before i swing. Now put this all together and should at least make some contact.
To bunt you should square around to the pitcher or partner and just lightly tap by (assuming you're a righty; lefties, the opposite) putting your left hand at the base of the bat, and the other hand somewhere near the neck of the bat (about 2-3 inches from where the grip ends). The right hand, at the top of the bat, acts as a pivot point. Someone on TV (forget who, someone famous) described bunting as catching the ball with your bat. Bunting may be hard at first but easy to get used to it.
Step 6: Base running
Stealing rules are different in different leagues. In mine, you can't start stealing until the pitch crosses the plate. The key to stealing is getting a good jump if don't get a good jump you may get out.
The in field fly rule is very confusing. Wikipedia has a good definition.
If you didn't understand my rules to tagging up wikipedia has a good description.
Step 7: Pickup games
To start one just call your friends up and start a game. You may want to try a pickup game; it might get you really in to baseball.
Thank you for reading my Instructable, and I hope this Instructable will prompt you to try baseball if you haven't! I like baseball because it is very fun, and is also it is very good exercise. Maybe you will too.
Step 8: Pitching
The first pitch you should learn to throw well is the 4-seam fastball. This is the pitch that is usually your hardest, and the one that goes straight. To grip it, take your first 2 fingers, space them about 3/4 to an inch away from each other, and put them so they are perpendicular to a seam. Hard to understand without pictures. http://en.wikipedia.org/wiki/Four-seam_fastball