it really depends on what you want to accomplish:
get lean, "cut," and drop some weight?? as suggested earlier, p90x will do it, especially if you're already in decent shape (strong). a family member does it religiously and he's probably around 5-6% bodyfat and obviously very "cut." however he's about 6'2" 175# and if you ask him to pick up something heavy and move it you're both out of luck. p90x has done great things for a shitload of people, and i'm not knocking it at all, but if you're wanting size i'd look elsewhere.
get stronger, bigger, etc. ?? compound movements with heavy weights. squats, deadlifts, cleans, benchpress, etc. perhaps look into crossfit ($$).
"increase my level of fitness" anything is better than nothing, and we all started somewhere. a 20-30 minute jog with some pushups, situps, curls, and pullups a few times/week will put you head and shoulders above most folks. stick with it consistently and you'll have something. if you don't mind dropping a little coin, a kettlebell, a sledgehammer and a tractor trailer tire will wear you out every time.
you cannot outwork a shitty diet.
hope this helps.