[radu_floricica]

What took me off my armchair some years ago, when I had really become a couch potato, was weightlifting. By chance a gym opened 2 minutes from my flat, so I paid them a visit. First the guy there showed me how everything works: he did a few repetitions, then I did a few repetitions (of course, at the lowest setting) on every machine there. After 10 minutes I thanked him, sat on a couch, begged for a bottle of water and spent the next 20 minutes trying hard not to be sick.

In a few months I was in reasonable shape, but you could see a big difference in posture (and in not getting sick) from the second visit. Kept doing it for about 6 months, then on and off another 2 years. Then I started doing other stuff, mostly for fun. Last thing was indoor climbing - most fun I ever had doing sports. Also very good at unexpected things: I never expected my balance to improve so much.

My point is: weightlifting is the best thing to start with, for two reasons. Time wise it's by far the most efficient, both as time spent per week (twice is enough), and as time until getting into shape. Second, it's very useful for whatever you want to do in the future. Whatever the sport chances are basic strength and a bit of endurance are required.