You pick the chord shapes you feel happiest playing for the piece, and then just count up the frets, with one semitone for each fret, and that's where yo put your capo

So if you want to use C shapes, to play in the key of F, F is five semtitones above C (C -> Csharp -> D -> Eflat -> F), so you put the capo just behind the fifth fret. If you wanted to use D shapes it only be three frets. If you wanted to use G shapes it'd be ten frets, and I wouldn't think you'd want to do it that way. (Though it actually makes quite an interesting sound.)