Make sure that you're getting a calorie surplus. You should usually eat 20 times your bodyweight (in pounds to calories), but I would also add 500 extra calories if you start working out.
If you only have dumbells, do bench presses with them - works your triceps and chest depending how you do it. Your triceps make up more of your arm than your biceps do, so it's the triceps that add more to the size.