Here is a free online guitar tuner you can use to tune your instrument. There are however some people who are going to have difficulty determining if the note from the online tuner is the same with the note from the string on your guitar. Developing a good ear to notes or tones takes practice and time. But there are those previledged few who have a natural talent for identifying and differentiating different notes and tones.
This online guitar tuner is for Standard Tuning (EADBGE), there are other variations or tunings but lets stick to tuning our guitars to standard tuning.
Click on the switch below each note letter to hear the note, it will continue to play at constant intervals until you click it again to turn it off. It is read from left to right, the left most E note represents your 5th topmost or thickest string going to the right most E note representing your 1st lowermost or thinnest string.
Listen to each note and tune your guitar's string accordingly, be careful not to over tighten your strings especially the last E string. In time you will be able to "memorize" the notes for each string, and you will be able to tune your guitar without the help of tuners.
Here is a diagram of how your guitar should be tuned to standard:
The bottom will be your 6th or thickest string going to your 1st or thinnest string on top.
If you the tuners do not appear you might need to download a flashplayer add-on for your browser here.