Scales to Play Over F#dim
Every chord is a subset of several scales: any scale that contains all of a chord's tones fits over it. Pick a chord to see the matching scales from its own root, the major keys the chord belongs to, and full fretboard diagrams for every scale on guitar.
A F#dim chord contains the notes F# – A – C. Any scale that includes every one of those 3 tones fits over it — from the root F#, that means F# Blues, F# Locrian, F# Half Whole Diminished, F# Whole Half Diminished, F# Gypsy Major and F# Gypsy Minor. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
F#dim is also diatonic to one major key: G major (relative minor Em). In a song from one of those keys you can also solo with the key's own major scale or its relative minor pentatonic across the whole progression, not just this chord.