I wouldn't do the testosterone injection unless you were tested as having low testosterone - the side effects are no good.
It takes a long time to build muscle weight - I would visit a site like bodybuilding.com and look up weight gain workouts.
Hi Jk, Try This Its a bit old fashioned, but worked for me back in the late 60s, a pint of full cream milk at breakfeast, next pint mid morning, the next one with lunch, againe mid afternoon, your next after training with 3 raw eggs in the galss and wisk up, just do this mon to friday, see how you get on.
Good Luck