Moscow Mule, if you would be so kind, Barkeep.

For the first time in my life I'm actually trying to be consistent with going to the gym. I had brief periods of time when I used the on-campus gym when I was in college but I always made the excuse of "being too busy" to keep it up. I'm still an underweight DYEL twink and have no idea what I'm doing but I'm trying my best.
I'm worried though. I recently turned 29 and have had the same body type my whole life so I'm not sure if I can do much in terms of building muscle. I'm not looking to be gigantic but at the very least I need some actual muscle on my bones and I worry that it will be harder with my age. Maybe I'm over thinking it too much.
I also worry I may be screwing myself by mainly focusing on arms/shoulders starting out, but those are the parts of me I hate the most so I'm trying to give them extra attention.