Don't lift at 12. Let your body develop some more. I think you should really start getting into weight lifting when you get into HS. And, don't try to build bulk muscle, build lean muscle.
You can get stronger by doing pushups for sure, or lifting something heavy, but not weight-related. Moving a heavy box or something is fine, but I don't think people do that.
Just stick to doing a lot of pushups daily. Maybe 100 a day, so 5 times a day, do 20 pushups.