The C sharp half-diminished chord is a 4-note chord consisting of the notes C#, E, G and B.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as C#m7b5.
Chords that are a subset of C#m7b5. These contain less notes but all of them are included in C#m7b5.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the C#m7b5 chord. Feel free to save or share the image as needed.