Paste the chords you've got. It'll tell you where the capo goes.
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A capo is a new nut. Clamp it on the 3rd fret and a G shape sounds a B♭ — same song, same key, three easier chords. That is the whole trick, and it is why the answer above names the shapes you play and the key the room hears.
Which fret? The one that leaves the most of your own chords on open shapes and the fewest on barres. That depends on the chords in your song, not just its key — so it counts yours rather than reading a table.
A ukulele has an open F that a guitar has not, and its body starts at the 12th fret where a guitar runs to twenty. Send a uke player up to the 7th for a blues in G and you have taken most of their neck away for two chords that already rang open. So the answers stop at the 4th fret and the chart at the 5th — which is the row that puts B♭ on F shapes, the one a uke player actually wants.
For the wall of a practice room. Each row is a key, and each fret next to it is a capo position that leaves the song on open shapes.