If you sign up at The Gym, you can get training by the staff, but be very sure to emphasize you want serious, hardcore stuff, else they'll put you on the namby-pamby stuff. Make sure they include squatting and deadlifting, that'll get things going right. (In good weather, they open the rear doors and have some trainees push a truck around - cool!) They've had award-winning bodybuilders train there.
There's a 24 Hr Fitness at Coit and Campbell, it's your typical cram-the-paying-clients-in shop, but has some good equipment. No idea if you can get any serious training at all.
If you're REALLY hardcore, MetroFlex is down in Arlington. Ronnie You-know-who trains there.
At 37, you're really not that old. Most of us here are in their 40s. I'm 55, Old Navy (another bodybuilder) is 67. No one, including ourselves, cuts us any slack because of age.
General rule: if you want to build some decent muscle mass, especially at our age, you must push, push, PUSH. Just playing with the weights won't cut it. As Derek points out, you have to eat protein until your belly cries Uncle and train like you have to win the powerlifting meet. If you're putting weight on the bar, you're going in the right direction.