There is several ways of receiving sound from your guitar:

1. Simply plug your guitar into your microphone connector on your soundcard Connecting your guitar directly into your soundcard is the most convenient way, because there is probably only minor noise to occur (on the way from your guitar to your soundcard). It is the most recommended way of tuning.
2. If your guitar has no output (like most of Spanish guitars), you have to get a microphone and connect it to your microphone connector. This also works well, but you have to avoid all unnecessary sounds in order to tune your guitar. For example, when somebody speaks to you when you are tuning your guitar, you will probably unable to tune it, because the signal will be distorted.
3. If you have no microphone, you can plug your headphones (from walkman, for example) into your microphone, but this way is not very recommended This is the worst way and should only be used when there are no other possibilities left. But anyway, if you decide to use headphones, be sure to hold your headphones pretty close to the strings on your guitar - to be able to catch the signal. If your headphones are big enough, they can act as a microphone. But not of very high quality.