To sit still, definitely your body needs to be conditioned
If you want to sit still, just working on your body is not good enough – you have to work upon your mind too. Particularly for this generation, focusing on settling the whole system – mind, emotions, body, and energy – is important.
To sit still, definitely your body needs to be conditioned – hatha yoga is towards that. But even if your body is in a good condition, you still will not be able to sit still, unless you settle some other aspects.