Start with the 30-Day Plank Challenge.<p>Since you mentioned good fitness, start recording what you eat in a food diary. This is the first step to eating healthier.<p>Also, get away from the damn screen and go take a walk.
Not to market this app but I highly recommend "Sworkit" app. Tag line: "No Gym, No Excuse"<p>PS: I did workout today using that app.