The notes of F#add9 are F#, A#, C# and G#. F# is the root, A# the major third, C# the fifth and G# the ninth. Said aloud, its notes are F sharp, A sharp, C sharp and G sharp.
From the low string to the high one you hear F#, C#, F#, A#, C# and G#. Of the six strings you play, two give F#, the note that names the chord. The lowest note is F# on string 6 and the highest is G# on string 1.
You play F#add9 with a barre at fret 2. Everything fits between frets 2 and 4. The second shape on this page starts at fret 9.
F# is one change away. String 1 moves from fret 4 to fret 2, bringing in F#, the root of F#. It shares two notes with D#m, F# and A#. Going to Bbm, two positions stay the same.
F#add9 belongs to six keys. In F# major it is the Iadd9, in B major the Vadd9 and in C# major the IVadd9. The rest appear in the key list.