Your arms, biceps, specifically, is one of the smallest muscles in your body.
But hence, everybody loves the huge bicep, It's what attracts attention and it's what everybody wants.
But tooo many people overtrain it. And therefore, they rarely get the results they want.
Depending on your schedule, i usually work my biceps with back or chest and the same with triceps.
Do not forget about your triceps. They make your arms look a lot bigger.
Just dont overtrain your arms.
Do some bench presses, some squats, some deadlifts, some shoulder exercises(military presses, arnold presses, etc), weighted dips, some bb curls or db curls, for me, I love preacher curls, they seem to work for me.
But thats the thing, find out what works for you. Get advice, go to bb.com and read some of their advice, visit some other boards, do some research and then go and do it.
What works for me, might now work for you. Yes there are only so many exercises, but maybe you need to do less or maybe you need to do a little more. Just go out and do it.
Also, you seem a bit skinny, for you to gain any mass and size, you need to start eating more.
To build muscle, you'll need protein in your diet.
Your arms are usually worked in every exercise. Be it shoulders, chest, back, etc. Your arms are always being used.
Some people don't even have an arms day and they can build mass. Some people say BB curls, etc are only isolation exercises. THey do deadlifts, squats, benches, and military presses.
I dont' know, but discover what you feel works best for you.