I've got thin arms how do I add some mass to them?
Mine are skinny too. If you're new to exercising like me I can strongly recommend beginning with a few simple exercises that cover the whole body, rather than zoning in on 'problem areas'.
Pull ups and pushups for upper body
Planks for core
Walking for legs (unlike the rest of me my legs are naturally quite muscly... don't want to end up looking like a T-Rex lol)
Keeps it nice and simple and results come surprisingly fast, arms included.
Oh and remember you need to eat right too...