In order to have a balanced swing, you must begin with a balanced stance. The weight should be on the balls of your feet and your knees should be slightly bent. Whether you are guarding a player in basketball or standing at the plate, you always want to be in an athletic, balanced position. Don't make the mistake of leaning your weight back on your heels. This is a poor position to start from and will cause balance problems as you stride forward and swing the bat.
In order to be balanced and in a good position to stride and use the lower half of your body I would suggest having a stance that is slightly wider than shoulder width. This will put you in good position to take a short stride and use your lower body to rotate and generate power. Your weight should be a little more on your back leg than your front leg. This will keep you from having too much weight transfer when taking your coil.
Just like how far apart your feet are in the stance, you'll see many players use a closed, square, or open stance when standing at the plate. For these lessons I would like you to start out with a square stance and then you can adjust from there after you've completed the lessons if you feel you need to.
One reason players will open up their stance slightly is to make it easier to get both eyes on the pitcher. If you are in a square stance and you find it difficult to turn your head far enough to get both eyes on the pitcher, then opening up your stance just enough to solve that problem may be right for you.
When we outlined gripping the bat we stated the importance of being relaxed. The same thing can be said for your stance. When you step into the box and get into your stance, your body should be relaxed. We don't want to step in the box tense or get into a rigid position while we wait for the pitcher to start his motion. Another common sign of not be relaxed is having "happy feet". This is where a player gets into his stance and then starts moving his feet around. This makes it very difficult to achieve consistent balance in your stance and can affect your timing. So when you get into your stance, make sure your feet stay in place.
Have your hands off your shoulder and at the top of the strike zone. This puts your hands in the best starting position for swinging the bat. It's very difficult for any hitter to catch up to a high fastball and hit anything but a pop fly if their hands start below the ball.