The notes of Badd9 are B, D#, F# and C#. B is the root, D# the major third, F# the fifth and C# the ninth. Said aloud, its notes are B, D sharp, F sharp and C sharp.
String by string, low to high, it plays B, F#, B, D#, F# and C#. Of the six strings you play, two give B, the note that names the chord. It runs from B on string 6 up to C# on string 1.
The main Badd9 shape needs a barre at fret 7 across six strings. Your hand covers three frets, from 7 to 9. There is one more position, starting at fret 7.
Badd9 and G#m have B and D# in common. Ebm shares two positions with this shape.
Badd9 belongs to six keys. In B major it is the Iadd9, in E major the Vadd9 and in F# major the IVadd9. The rest appear in the key list.