Here's a little ditty I wrote (based on "My dog has fleas") to help students solidify the tuning pitches in their ears. Sing it first and then pluck strings on "my dog has fleas" as you sing. A teacher may add the optional chordal accompaniment.
Here are some links to great online ukulele tuners:
YouTube Ukulele Tuner 1
YouTube Uke Tuner 2
Pineapple Pete's Uke Tuner