Top spin is useful for rallying under pressure and finding angles you can't find with a flat shot. It's not the answer to everything in tennis though. If you have good, flat shots already then you can become a very good player with just those.
As for producing top spin, there are several coaches on here who can probably give you better advice than I but I'll just say that your swing is very similar to that when you hit a flat shot except you aim to brush up the back of the ball rather than hitting through it. It will take a lot of practice and you'll end up skying a few balls at first. Just remember to start off slow, and then once you get the technique start adding direction. Once you have the direction then you can start adding power.