Don't worry about curls so much. It could just be a case that your forearms are weaker than your biceps so tire more easily. If you are curling the weight properly your biceps have to contract in order for the weight to move, theres no two ways around that.
Do you follow a complete exercise program right now?
I would read the stickies at the top of this Training forum for some great beginners info about exercising with weights.