When I bought my first guitar lo those many years ago, the store also had a giant poster for sale that labeled every fret with its note name. Within seconds I went from "I'm gonna be a rockstar!" to "Holy crap that's a lot to remember! Maybe I'll study physics instead."
Those stupid posters will do nothing but give you a migraine. But you still need to know where all those notes are. Lucky for you (and your head), there's a magic bullet.
There are three simple steps to being able to locate every note on your fretboard, and just a couple little things to memorize. Let's get to it.
Step 1: Half Steps and Whole Steps.
A half step in a movement of a single note, one fret on your guitar. A whole step is, as you've probably already guessed, a movement of two frets. In this step we're going to find out where the half and whole steps are in a C major scale. The only reason we're using C major is because it has all natural notes (no sharps or flats). The scale itself doesn't matter all the much right now.
Here's a C major scale: C D E F G A B C
Memorization Point #1: The only half steps are between E and F, and B and C. Everything else is whole steps.
Step 2: Learn the bottom two strings.
First learn the natural notes on the bottom two strings. Do this by starting from the open string note (E for string 6 and A for string 5) and use your system of half and whole steps you learned to find out where the rest of the notes are.
You can check out the figure below to help you. But you'll find it sticks better in your brain if you figure it out on your own. Use the figure just to check your work.
Practice this in two ways. First go in alphabetical order forwards and backwards on each of the two strings. (Just the 5th and 6th strings! No need to do this on the rest. We'll get to those later.)
Once you're comfortable with that, give yourself random notes to pick out on each of the two strings.
Now we can get to the sharps and flats. A sharp note goes up a half step (one fret) from the natural. A flat note goes down a half step. Go back through strings 5 and 6 giving yourself random sharp and flat notes to learn those.
Step 3: Read the rest of the fretboard with three simple patterns.
At this point, you've learned the 5th and 6th strings. You also know the 1st string because it's the same exact notes as the 6th, just two octaves higher. But all the letters are in the same place.
Here's where the magic is. Instead of busting your tail working out the half steps and whole steps on each string, you can use a couple simple patterns to figure out everything else.
Start from a G (just as an example) on your 6th string. Now go up two strings and up two frets. That takes you to a G on your 4th string. That patterns works up and down the whole neck. Simple! See the figure below for the shape I'm talking about.
Now grab the D on your 5th string. Same pattern. Go up two strings, up two frets. That will take you to a D on your 3rd string. Splendiferous! Again, it works up and down the fretboard. The figure below gives you the pattern.
The last pattern changes just a little bit. Grab the G on your 4th string. You can find that based on G on your 6th string like in the first pattern. Now to get to the 2nd string you'll go up two strings and up THREE frets. That will take you to the G on your 2nd string. Stringalicious! (Ok, going too far now...) Check the figure below if you have trouble.
You can actually use that same up two/up three pattern to get from the 3rd string to the 1st. But as I mentioned earlier, your 1st string is the same as your 6th. So you don't really need to memorize that extra bit of info.
Memorization Point #2: The three patterns in these examples.
Now you've got the whole neck! Practice this in two ways. Give yourself random notes (don't forget the sharps and flats!) and find the note on every string using your patterns. So do, 6-4, 5-3, 4-2, 1. When you're comfortable with that, do the notes again but on adjacents strings. This forces your brain to switch patterns for each string which is slightly harder. But it's also a more real-world way you'll be looking for notes.
There's another magic bullet you need to be aware of for guitar. Especially if you're just starting out, but even if you've logged some string time already. And this magic bullet is more important than anything you've read so far. Click here to see what it is.