The Thickest string is called the E string or the 6th string, which is at the top of the guitar, when you hold it in position. The string below this is the A string. Below this come D, G, B and finally the E string in that order.

It would be surprising to note that both the extreme strings are called E strings, however one is thin and the other is thick. Theoretically both these strings produce the same pitch, although there maybe variations in tone.

There goes a saying that helps you remember the names of these strings.
"Eat All DayGrow Big Easily", where the first letters of each word correspond to the strings E, A, D, G, B and E. Just say it out loud once or twice.