The C# note is the black key immediately to the right of the C note as shown in the diagram.
You now know enough notes to play the D major scale. The D major scale begins and ends on a D note and contains two sharps - F# and C#.
The # signs after the clefs indicate that all F and C notes on the stave are played as F sharp and C sharp respectively.
The C# note is the black key immediately to the right of the C note as shown in the diagram.
To play the A7 chord use the first, second and fifth fingers of your left hand.