Enjoy Tunessence's free online guitar tuner. It works for both acoustic and electric guitars.
Play your lowest E string, and the tuner will tell you how far flat or sharp you are. Adjust the tightness of the string with the tuning keys on the head of your guitar, loosening the string to make the pitch lower and tightening it to make the pitch higher. Once the meter is right in the middle, the string is in tune. Repeat this process for each string going up, until you’re in tune.
Still not sure how to use this thing? Then learn how to tune in more detail!