If you know how to play guitar, then will you know how to play a banjo, or another similar instrument?

Banjo and guitar aren't the same -- the strings are tuned differently, the chord shapes are different, the right hand picking is different -- but if you already play guitar, you already understand chords and picking and music theory and such, so it will be easier for you to learn banjo than if you had NO previous fretted string instrument background.