If it looks similar to this one, and you have an electric guitar, then you will need to plug your guitar into the INPUT socket. Most electronic tuners these days work pretty much the same. The lowest string on your guitar is going to be an E.. you should have some idea of how low the lowest string sounds like already, so get it to that approximate tuning position first, then pluck the string string and look at the gauge of the tuner. If the needle is to the left of the middle, you need to tune the string higher in pitch, if the needle is on the right of the middle, then you will need to tune the string lower in pitch.
You can see which note you are playing on many electronic tuners, the one in the picture is even showing which string it is.. in this case the 5th string which is an A note. The best way to proceed from this point, is to play the 5th fret of the 6th string and then tune the 5th string to that same note. Once it is approximately the same, use the tuner to get it exact if you want it exact. Do this for all the strings except the B string (2nd string), which as FretSpider pointed out, you will need to play the 4th fret of the 3rd string to get the approximate pitch/tone for.
Always remember that as you increase the overall tension of the strings of your guitar, you may affect the tuning of the other strings and they may need re-tuning. Also.. with a new set of strings, they will go out of tune a lot until they have been stretched or worn in...
Anyway..
* If you need it to be exactly the same as a record/mp3/CD you are playing along to, then you will need to use your tuner;
* If you DON'T need it to be exactly the same as a record/mp3/CD, then you can just tune all the strings to the approximate tunings by using the relative tuning instructions.. where you tune them all off the 5th fret of the previous string except the B string which you tune off the 4th fret of the previous string.
