Obesity and climbing are a tough combination. I would recommend trying indoor climbing at first and working on routes that don't overhang (wall is <= 90 deg from the angle of the floor). If you work this in with a routine like cycling/runnning/swimming and a proper diet (start eating less carbs unless you plan on working out within the next couple of hours), you can develop a body built for climbing while picking up proper technique at the early stages.

Source: I've been climbing for 6 years