when my opponet is at the mound he always throws balls but i always swing how can i prevent that

One of the best tactics is to guess the pitch. The most obvious pitch to guess is the 4 seem fastball. Guess the pitch and wait for the one thats in the strike zone.

Now, as far as watching for balls. You'll see enough and learn when it looks like its going to go out of the zone. It will just come over time. You might want to try adjusting your sliders for pitch speed, and you can watch the pitches better until you get the hang of it.