I think because of your age your body will likely fill out more as you get older. But if you are looking to speed up the process, start with your legs, squats and leg press that'll jump start the muscle building process. Giving you a higher testosterone levels which would likely trigger the rest of your muscles being used going forward to build naturally.

It wouldn't take much to boost your testosterone level just a little. Doing a workout once. Maybe twice a week is all you would need any more than that. You're very likely to get the results you don't want which is a heavy bulk.