Find a teaching pro, in person or on-line, and get a lesson. They can look at a video of your swing and give you some changes to work on that will very likely make a big difference. The basic cause of a slice is that the club face is "open" relative to the swing path. This creates spin that causes the curve (like in ping pong, if you ever played). Your hands control the club face and your arms, upper body and foot position mainly determine the swing path. Lots of variables and a trained eye can help you find ways to improve.