Thanks to the advent of guitar tablature, amateur guitarists are largely spared the trouble of learning to read classical notation. (Don't expect that to fly in a professional context, though!)
What we do need, however, is to understand enough to fill in the gaps that are left by tabs. Most notably, that's the timing.
At TheAxe.ca I typically include both classical notation, and tablature for a piece. This is to give us the best of both worlds, tablature for easy reading, and classical notation for the exact information.
Here we have a collection of notes of different length, color coded for easy reading. In bars 1-4, we have them as standard notes, and in bars 5-8, repeated as rests.
Make sure that you can identify the length of this notes and read them at a glance! As you can see, the tab does not give you information on the length of the notes. At best, it has to be inferred, often by listening to the song.
Red notes are whole notes.
Orange notes are half notes.
Yellow notes are quarter notes.
Green notes are 8th notes.
Blue notes are 16th notes.
Here I've highlighted the time signature.
This tells us the length of the bar, and continues throughout the piece unless we receive a new time signature later in the song.
A vast majority of popular songs stay in one time signature the whole time.
To put it as simply as possible, 4/4 and 3/4 time are going to be all you need to worry about for almost everything you try to learn.
In 4/4 time, we count 1, 2, 3, 4, then repeat. That's 4 quarter notes.
In 3/4 time, we count 1, 2, 3, then repeat. That's 3 quarter notes.
Lastly, I want to mention dotted notes and triplets (highlighted below). These are modifiers attached to notes to alter their length.
A dotted note is 50% longer than it would normally be. A half note is normally two beats, so it becomes three beats when dotted.
A quarter note is normally one beat, so it becomes 1.5 beats when dotted.
Instead of lengthening a note, a triplet modifier will shorten it. When you make a group of three notes a triplet, you shorten the length of them to fit into the space two of those notes would normally take up.
So, three half notes in triplet form equals the length of two regular half notes.
Three quarter notes in triplet form equals the length of two regular quarter notes.
This is one of those things that can be hard to get your head around at first. I promise in time it will become second nature!