Sharp
A sharp (♯) is a sign that raises a note by a half step: one key up, to the right. A sharp before F turns it into F♯.
In written music the sharp stands right in front of the note, on the same line or space. It applies to that note until the end of the measure; in the next measure the note is plain again unless the sharp is written anew. A sharp placed right after the clef, in the key signature, applies to the whole piece, in every octave.
Most sharps lead to a black key: C♯, D♯, F♯, G♯, A♯. Each black key also has a second name with a flat, so F♯ is the same key as G♭. In Lithuanian and German letter names a sharp adds the ending “-is”: Cis, Fis.
From E and B a sharp leads to a white key: E♯ sounds as F, B♯ as C. A sharp is cancelled by a natural sign.
What it sounds like
A sharpened note sounds just slightly higher than the plain one, by the smallest possible step. Listen closely: F and F♯ differ, but not dramatically.
Where it is taught: the studio’s interactive textbook, chapter 5, “Half steps, whole steps and the black keys”.