Tuning a Cello using a Piano
Tuning your cello using a Piano or Keyboard is a simple task. It is pretty easy and good for beginners because your only playing open strings on the cello, so you can have a free hand to turn the pegs while the piano and the cello are resonating. It's also better because you can tune each individual string, instead of getting one reference string and tuning the rest of the strings to it.
Open Strings - Down below I will talk about playing Open Strings. To play an Open String means to play a string without fretting anything. So that's why it's called Open.
How to Tune a Cello with a Piano
The picture above shows you each string on the cello, it's same note on the Piano or Keyboard. Just follow the picture for each individual string and tune the string until it matches the keyboard or piano. Just play each string open, you don't have to fret anything. Make sure you use the 'Middle C' on the picture as a reference point so you don't tune your cello an octave too low, or too high.
Congratulations on tuning your cello!