If you biceps are naturaly uneven, then working your biceps evenly over time with sort them out.
If they are growing unevenly because of working out them its your technique. You may be unbalenced, be leaning to one side or have your hand in the wrong place. using a bar or bicep machine can cause your stronger arm to take the weight, thus your work it harder.
If you have one weaker arm, improve your technique (ask gym staff, or members, or start a topic for specific questions)
If your technique is good then you'll have the same amount of resistance on both biceps. This means your weaker bicep will be worked harder.
A good way to make sure your stronger arm isn't taking the weight is to use dumbbells.
Bad muscle symmetry is rarely due to weightlifting and most problems with sort them selfs out in time.