Pretty much every guitar tuition site will give top class training in using a capo, but just to be safe you can check out the best online guitar lessons reviews to see whats available on the market.
How to Use a Capo
You need to place the capo behind one of the guitar's frets. If you will place the device on a fret, the strings will produce undesirable sounds. Make sure that your capo is not exerting excessive pressure on the strings. Guitar strings go out of tune when they receive too much pressure.