Alright, so you're ready to start figuring out the note names for each string of the guitar. Great! You might be a brand-new guitar player, or even an advanced "ear" player, who'd like to get around to learning how to read some notes on the guitar. Either way, welcome, and happy that you can join.
The thinnest string on your guitar is the high E string. It's tuned to the note "E" (and remember there are 7 note letters.. A, B, C, D, E, F, and G). That means, when you pluck that "E" string, assuming your guitar is tuned properly, the sound that will emanate will be a certain frequency that's grouped as an E note. (There are different "octaves" of E notes, so it's just part of that family of E notes.)
Next, here are the 3 notes you'll now learn on this high E string. Which by the way can also be called the "first" string. Pluck the open string to sound out the initial E note, then fret the 1st fret to hit the F note, and fret the 3rd fret to hit the G note.
By holding down your guitar's string against the metal bars (the frets), you're "stopping" the string, shortening its vibrating length and thereby changing its pitch. In Western music nomenclature, that open string pitch of the 1st string is the E, the first fret sounds out the F, and the 3rd fret sounds out the G.
The first note, the E note, is played on the "open" string; that means you don't actually fret a note - you just pluck the top "E" string, open.
The next note, the F note, is fretted on the first fret of this thin 1st string. Use your index finger for this fret, and in general try and keep the principal of assigning your 1st finger to the 1st fret, 2nd finger to the 2nd fret, 3rd finger to the 3rd fret, and 4th finger (pinky) to the 4th fret, when playing in open position.
Finally, the G note is fretted on the third fret of the 1st string. This will be played with your ring finger - the 3rd finger.
Now, using the music staff diagram above (as a guide), read the lines of music below.