I recently bought a cheap used department store road bike. I wanted to get into the sport for fitness but cannot afford to spend a lot of $. So far I love it and it works great for what I need it for. My only issue is that the previous owner did not care for it very well and there is a fair amount of rust present on the cassette and chain. The chain I can deal with but what is the best way for me to care for the cassette? Don't really care about aesthetics just want to stop the problem from getting worse and affecting the performance of the bike. Also what sort of lubricant should I be using?

Thanks.