Key signatures indicate the key of an entire piece of music or a section of it. Therefore, they are always written directly behind the clef (for the ukulele, always the treble clef: 𝄞) and before the time signature.
Possible signatures are ♯ (pronounced: sharp) and ♭ (pronounced: flat). The signatures apply to all affected notes of a piece, unless they are marked with accidentals.
Accidentals apply to all subsequent notes within a measure, and in the case of slurred notes, also beyond the measure, regardless of the string on which they are played.
Revocation of previous accidental
The Ukulele is always chromatically tuned, that is, the distance between a fret and the next is a half tone step. It is therefore not possible to play quarter-tone steps on an Ukulele (except approximately by bending).
On chromatically tuned instruments, the smallest possible step size is the halftone step. As a result, a sharp note is identical with the flat of the neighboring natural, when the natural is one full step away, or the next natural itself when it is only half a step away. G♯ is therefore identical to A♭, B♯ to C. The reverse is also true: C♭ is identical to B.