How can I get slimmer arms?

I have a problem with my arm flab lol. I'm 5'7 and 125 pounds, I workout everyday. Well when I say workout I do cardio, mostly running daily, some kickboxing, dance, bike riding, etc. I don't lift weights. I am wondering if I should start. I have always had kinda big arms in proportion to my body, they have muscle underneath the fat too, its not all fat. If I lift some light weights will they get even bigger? I want nice slim arms. Please I need advice.