What is a good arm exercise i do not want to build muscle i only want to tone my arm?

Exercise alone will not build muscle size, you need to be eating a calorie surplus to do this. Toneing is a function of fat loss, you need to be in a calorie deficit to loose the fat covering your muscle while doing strength training to maintain your existing muscle.

To develop your arm muscles you need to work your Tricep and Bicep and Brachialis. To keep a good balance and avoid injury a full body workout is recomended.

Tricep Exercises

Close grip Bench Press
Tricep Pushdowns
Tricep extensions
Skull Crushers
Tricep Kickbacks

Bicep Exercises
Bicep Curl
Supine Curl
Incline Curl

Brachialis Exercises
Preacher curl
Concentration Curl
Prone incline Curl