Before you begin playing a guitar you should be sure that it is tuned correctly otherwise the sound that comes out of it might not be the one you wanted to.
Strings are usually tuned in the following order E A D G B E which can be remembered as Every Apple Does Go Bad Eventually or the other way around (E B G D A E) as Every Banana Gets Digested After Eating. The first string is the one that should be tuned to the highest frequency 329.63Hz which is E4 or the tone that is major third above the middle C. The second string is B3 just (minor second) bellow the middle C which is tuned at 246.94 Hz, the third is G at 196.00 Hz, the fourth one is D at 146.83 Hz, the fifth one is A at 110Hz (440 divided pay 2 power 2) which means is 2 octaves below the A that serves as a standard for musical pitch and the last one is E2 (82.41Hz).
If you don't have an absolute pitch which is pretty rare you should be able to tune the guitar with the help of piano or even computer that will play you those tone described above. But if you have a lot of problems with it the best way is to get a guitar tuner which is a device that will help you find the right frequency that your string plays. The cheapest way to do it, is to simply google for guitar tuners and you will find many websites where you can hear E A D G B E tones and tune your guitar.
You should tune your guitar almost every single time before you play it, guitars go out of tune pretty quickly specially if the temperature and humidity is not constant. If you are a beginner don't be worried about tuning, once you will do it several times you will quickly hear if your guitar is not tuned correctly and you will be able to tune it in a matter of seconds.
Learn more about tuning and how to play guitar at Learn to play guitar.
