C sharp (noted as C# in musical notation) tuning is a non-standard guitar tuning. It is a step and a half lower than standard tuning. By tuning each string to the appropriate steps down, when the strings are played open, a C# chord will sound.
Make sure that the guitar is in standard tuning and is in tune with itself. Tune the guitar strings to EADGBE (6, 5, 4, 3, 2, 1, from thickest to thinnest). This should be the standard setting on your tuner.
Tune the bottom string (6th) down three half steps from E to C#. If you have an electronic tuner, hit the "flat" button three times. The tuner will then tell you where your pitch is relative to C# rather than E. If your tuner does not have a flat button, tune down to D using the tuner, and then tune one more half step down. You can play a C# elsewhere on the fretboard for reference (6th fret, G string).
Tune the 5th string three half steps down from A to F#. Use the flat button on the tuner, or play an F# (fourth fret on the G string).
Tune the 3rd string down three half steps from D to B. Your tuner can help with this even if you don't have a flat button.
Tune the 4th string down three half steps from G to E. Use the tuner to help you find E.
Tune the 5th string down three half steps from B to G#. Hit the flat button three times or play the fourth fret on the high E string for reference.
Tune the 6th string down three half steps from E to C#. Use the bottom E string for reference. Play the six strings open. A C# major chord should sound.
Tune the strings carefully and if possible use a heavy gauge so that the strings will not break.