Hard to say without seeing your swing but there are a couple of things you can check pretty easily.

1. Your grip: If it is too weak you will slice. Try rotating your left hand so more knukles are showing.

2. Out to in swing path: This means you will cut across the ball imparting sidespin on to it. This is harder to get rid of but ther are a few good drills you can find about the place to help you get rid of it.

Sam