Re: gain some muscle(arm area)
Arm exercises with high reps and low weight sound like what you're looking for (the lean muscle approach).
Do reps of around 20 or more with low weights between 2 to 5 pounds maybe, maybe more if you can handle it. Work the biceps the triceps and maybe the wrist/forearm (but not too much since these are trained with the biceps/triceps anyway and depend partly on genetics too).
You can find many bicep/tricep exercises on the net. I suggest you do some shoulder exercises too.