No matter what musical instrument you play, there are only 7 notes that you can choose from, these notes are given names from the first 7 letters of the alphabet: A through to G (known as natural notes).
The six strings on the guitar running from thickest to thinnest are played openly (without fingers on the fretboard) are given a name from these letters, and they are also given number to tell you which string you should pluck.
The name of the note played and the number of the string running from thickest to thinnest are illustrated below:
E – 6th string (thickest)
A – 5th string
D – 4th string
G – 3rd string
B – 2nd string
E – 1st string (thinnest)
As you can see the thinnest string is called the 1st string, and the numbers go up as you increase in thickness. You must memorize this as this is very important and you will come across it often.