You definately need a cup for free sparring. The first time you get kicked in the groin, you will know why. Don't get a cheap one though, I've seen some broken ones !! When do you get strong ? Well, it depends. In my experience, you get stronger when you really learn to use your body korrekt in techniques. E.g. hiprotation, korrekt tension and relaxation to name a few. Of course, sit ups and push ups helps too.