Use this free online viola tuner to tune up your viola. This is for the standard CGDA viola tuning. Click on the note for the string you want to tune and then turn the tuning pegs on your viola and match it up with the note that is being played. Use your tuning pegs at the top of the viola to get close to the correct tone, and then use the tuning pegs on the bridge to do the fine tuning.
Note: Both the Viola and Cello have the same tunings, only the strings on the Viola are 1 octave higher. So you can use this online tuner to tune your cello also!