How do you choose which strings to use?

Both electric and acoustic steel strings go from a heavy to a light gauge. A light gauge is better for faster playing on electric guitars. On an acoustic, a light gauge is better for fingerpicking because it is easier on your fingers and less force has to be applied when playing. A heavy gauge on a steel string is better for heavy riffs and electric riffs. Nylon also goes from low tension to super high tension. Certain guitars are made for a low-tension feel and some guitars are made for a super high-tension feel, like classical versus flamenco. Classical guitar will most likely have medium to low tension, depending on the player. Low tensions are easier to play, but they also bend very easily and you have to be careful of intonations. If you're playing super high tension, it's going to be a much firmer feel on the guitar and it will respond more because of the extra pressure. Every guitar is made differently and certain strings only fit on certain guitars.