If you watch a baseball game at the highest level, you will notice that every batter has their very own personal and unique look about them while they are hitting. But what you don't see, is that while they may stand differently, or hold their bat in different places, they all perform the 5 key components in the same way. I say 5 because stances definitely vary according to the player. I will include instruction on the stance just as a generic way of having a starting point. The 5 key components are: 1. Balance 2. Load 3. Stride 4. Bat plane 5. Contact point. Learning to do these 5 things in the correct manner will put you a step above the competition.
Now, stances vary per player, but a good way to start out is just stand with your feet a little wider than shoulder width apart, and slightly bend your knees. It's important that you are bending your knees, and not bending over at your back. Now, make sure that your feet are facing at a 90-degree angle towards home plate, and your front foot should not be closer to home plate than your back foot. Now, turn your head to focus both eyes on the pitcher, while holding your bat about 6-8 inches away from your back shoulder. Both elbows should be loose and relaxed, and not sticking out. Just be comfortable; tightening up will only take away from your swing.
Now, you must have good balance in order to take a good swing at the ball. If you are falling all over the place, a few things will happen. First, you won't be able to see the ball properly: it's going to look like it's moving all over the place. Second, you will not be able to hit the ball hard at all. A good place to start is to have your weight on the balls of your feet (just below the toes, but not on the toes), with your knees slightly bent. You should have your weight evenly distributed on both legs, so that you are not leaning forward or backward. Think of yourself as trying to get into a position where you are going to allow someone to try to push your chest and knock you over. If you are standing tall, and your weight is on your heels you are going to tip over backwards.
Now that you are in a good position to hit, it's time to start your swing. You do that NOT by moving your hands, but by shifting your weight from 50/50 on both legs to about 70-80% on your back leg and 20-30% on your front leg. But you are not going to hold it there. This is called your load. You must pick the right time to start so that you don't load and then stay in that position. A good time to start is when the pitcher has his leg coming forward towards you, and he is about to bring the ball forward to throw. Anything earlier than that will have you out in front, and anything later than that will have you behind the baseball. This load component leads right in to the next step, the stride.
After you have started your load, you must transfer your weight forward, past your starting point of 50/50 weight balance. So you start at 50/50, then load back to about 70-80/20-30, and now you take your step with your front foot, nice and softly, and have 60-70% of your weight on your front leg, and 30-40% on your back leg. You must make sure that you are not just leaning forward. You want this to be an actual step, with good balance, and your chest NOT leaning forward. Your upper body should be in the middle of your legs, and your momentum should carry the proper weight to the front leg. Also, as you start your stride, you should push your bottom hand back slightly (maybe 2-4 inches) towards the catcher, in order to give your hands some momentum towards the ball. Now, when you stride, you should NOT bring your hands forward- we will talk about that in the next step: Bat Plane
Alright, you have taken your load, and have pushed your bottom hand back towards the catcher slightly, and have taken your stride step. Now, in order to get some power into your swing, you must have the proper bat plane. In order to do that, once the heel of your front foot hits the ground on your stride, you must turn your hips. in order to do this, you have to make sure your back foot turns 90-degrees: from toes pointing to home plate to toes pointing towards the pitcher. You can practice this by getting in your stance, and just turning your back foot 90-degrees. Try it about 10 times. Once you have taken your stride, this process will be a lot more natural, and much easier to perform. Now, as you turn your hips, your hands will automatically move towards the hitting plane. You shouldn't move your hands yet. If you move your hands during your stride, you will be out in front of the ball, leading to a pop up, or you will come around the ball, resulting in a lazy ground ball. It is important to wait until after you have started your hip turn to move your hands. The direction your hands should go can be explained like this: you want to get the barrell of the bat to the ball in the shortest distance and time possible.
Finally, the contact point. Now, this will differ based on the location of the pitch. For a pitch right down the middle, you would look to make contact when the ball is about at your front foot (after stride). For an inside pitch, you want to make contact more out in front, so you don't get jammed. However, it's important not to come around the ball in order to get to that pitch. For a pitch on the outer part of the plate, you want to let the ball travel closer to the catcher. This would be a point about even with your back thigh.