One half step below the note. In other words; to go from D to C would be a whole step, to go from D to D flat would be a half step.
That would be either called C# or Db.
C-sharp. The leading tone is one half-step below the tonic.
Generally, the sharp and flat signs are known as accidentals. They alter the pitch of the note. As an example, D sharp is a half-step higher than D where D flat is a half step lower.
An interval is the distance between two pitches. These intervals are measured in half-steps and whole steps. For example, a half-step is like C to Db. A whole step would be C to D. A major scale is made up of these steps as so: C MAJOR Whole step, Whole step, Half step, Whole step, Whole step, Whole step, Half step. C to D, D to E, E to F, F to G, G to A, A to B, B to C WWHWWWH You may have noticed that from E to F and from B to C it was a half step just as if it were from C to Db. This is because these pitches are simply a half step away from each other.
half a step down, otherwise known as open d# tune all the strings down half a step
F sharp
There are two half-steps in the key of b: D to E flat and A to B flat.
If you are asking about the interval it is a minor third (or an augmented second...) In a Chromatic scale starting on C the tones are: C C#(or Db) D D#(orEb) so the whole step (two semitones) would take you from C to D, and then the half step would get you to D#(or Eb... they are enharmonic equivalents, which means that it's the same pitch with two different names)
2 sharps is D Major or b minor. The two sharps in this key are F# and C#, so you just have to look at the second sharp (C#) and move up a half step for the key (D). To find its minor equivalent, move down three half steps (3 half steps below D is B natural).
It's one half-step, or one semitone.
You simply move the key down one half step