In standard tuning, the open string pitches are:
E A D G B e (low to high)
This is easy to memorize with the acronym:
Eat A Darn Good Breakfast early
The strings are often referred to by their open-string note names but can also be referred to by numbers (low to high):
6 (E) 5 (A) 4 (D) 3 (G) 2 (B) 1 (e)