Hold the bat slightly above your shoulders, bend your knees, and when you feel the ball is at a perfect place, hit it.

First you need to find a stance that is comfortable, this really does not matter in your swing, what matters is where you start your swing from, meaning your loading phase. This when a hitter will slightly take his hands back towards his back shoulder at shoulder height. This where the swing begins. Some hitters having different stances but all of them start their swing from the same position. The "load phase" begins with your stride. The stride is a key point to hitting. It is used to generate momentum towards the ball. If you stride too far, too short, to the back of the box or towards the plate then you become unbalanced and will most likely take a bad swing. A good stride is straight to the pitcher with your front foot a little more wide than your front shoulder, with your toes pointing towards the pitcher or at a 45 degree angle. Once that is established then you can move on. After your stride foot is down then you can start you swing which is going to involve your hands and back hip and leg. Your back hip should "pop" towards the ball, meaning turn on your back foot like you are squishing a bug under your foot. This will allow your hips to get involved. Your hips and legs will aid in the generation of bat speed. Now you have the lower half where it should be and your upper half should not have moved much. From your load position, you should swing your bat to the ball, by using your hands not your arms. If you use your hands this will make the bat like a whip which will help with "pop". Your bottom should pull down towards the ball, leading with the knob of the bat. Your top hand will follow and punch through the ball when it is time to hit. The palm of your top hand should be facing towards the sky, while the palm of your bottom hand is facing the ground. The swing path should be directly to the baseball at a angle similar to chopping wood. With this swing path you will generate back spin which will make the ball rise and carry further. Where is your head at this point? It should be down looking at the bat, the ball and at the point of contact. You finish naturally not stopping your swing. The greater the extention is at the end of your swing the better chance you have of getting lift. You should end up with the bat going over your front shoulder and your top hand staying on as long as possible. Very similar to a follow thorugh on a golf swing.

Most hitters use a method called the A-Frame. You stand comfortably with your feet about a foot apart holding the bat so both sets of knuckles are aligned. Your hands should be right next to your right shoulder for a right-handed hitter with your elbow kept up enough to keep the forearm level. When you see the ball you need a "trigger" to start your swing, for most batters it's a slight lifting or sliding of their front foot as they start moving it forward to step into the pitch. I use a slide of the foot forward only a few inches, it's all about slight movements. Then the body swivels around as you swing into the pitch. Coaches emphasize "hitting with your hip" since you need to "swivel" on that back hip and it should feel like you're hitting the ball with your back hip. Extend the arms into the swing. A lot of the weight should stay on that back leg. During the swing your body at the point of contact with the baseball should form roughly the shape of an "A", the reason for the hitting style's name.

This is an excellent web site on hitting a baseball, with video. http://www.batspeed.com/mechanics.html