Weight loss!?

Just about 5 months ago I decided to try a diet plan. The diet I chose was the Atkins diet and I can honestly say within those months I got results. I'm 21 years old and a guy and 5 months ago I wore a size 51 in jeasn/shorts and a double X in shirts. Now I wear a size 46 in jeans and 45 in shorts and only wear X large so I was excited that I got results. I know it probably doesn't sound like much but I can tell a big difference and so can everyone else. Now I'm trying to find the right kind of excersises to do along with it that wuld help and was wondering if anyone had any advice? Would joining a gym be good to do also? Does anyone have any advice?