My first post has been inspired by my first visit to my new local gym. I was downstairs in the free weights room with 8 other men, and 6, that right, 6 of them were doing arm curls. Now don't get me wrong, at the end of a weights session it always feels good to do arm burnout, but to improve the size and strength of your arms, you need to do large, compound exercises (exercises that use multiple muscles). These exercises will ultimately be targeting other muscles but you will also need to use your arms indirectly to perform them.
Simple examples of exercises would be
Dumbbell bench press
Seated shoulder press
Chin ups (or lat pull downs if this is too difficult)
Bent over Row
Start with performing 3 sets of 10 on these exercises with a 1 minute rest between sets. You should be working hard enough for your muscles to 'fail' on about rep 8 of the 3rd set. If your muscles do not fail, you are not working hard enough and your body will not adapt and improve.
When you have completed this, feel free to do arm curls until you cannot lift your arms!!