Certainly not an expert but I'll take a stab at it, depends on what you mean. To change the key you place the capo on the desired fret to match the key you want. Standard tuning on a guitar is E if you place it on the first fret you raise that to F. That's one half step. There are no sharps or flats between E and F. If you now play an E shape chord you get F. If you place it on the second fret you raise the the key to F#. Play the E shape and you get F#. Third fret G, play the E shape you get G and so on. Remember when you get to the B position the next half step is C, there are no sharps or flats between B and C.
Its kind of like playing barre chords without having to bar across the strings. The capo takes the place of the barring finger.
The good thing about the capo is that you can now use all those open chord fingerings and get different chord sounds.
For instance capo on second fret play the open G shape and you get A. PLay the C shape and you get D. Play the D shape and you get E.
Theres lots more to it and there are many folks here that will explain this better than I can and correct anything I get wrong.
If you you were just wanting to know where in the fret box to put the capo, most people suggest puting it as close to the fret wire as possible but trial and error will be your friend on this. I have big fingers so I have to move it back a little so my finger will fit.
One more thing, it is probably a good idea to retune your guitar once the capo is in place.