There's a metric megaton of stuff online about scales, scale theory and what works with what. The difficulty you're going to have with online, mostly free, stuff is quality control, as some online contributors explain simple things in an unnecessarily complex fashion, and some aren't any good at explaining the advanced stuff.

Being old enough to have started learning without the aid (or distraction) of the World Wide Tubes, I started out with "The Guitar Case Chord Book" and shortly thereafter "The Guitar Case Scale Book" (sadly I can't say who the publishers are because I've leant them to a friend), which I'm sure would still be in print. They're no-nonsense but they are rather clinical, and I did, for many years, harbour the honest hope that the Scale Book would enable me to unlock some kind of "key" to music and my understanding of it... no such hope: I ended up spending far too long thinking about WHY scales are they way they are, without ever working it out, until I realised that they just ARE and I should just get on with learning them.

If I were you I'd start with learning your standard 2 octave major, harmonic and melodic minors, and pentatonics of the same. If you understand the CAGED system start in the E shape, then the A shape, then the D shape. G shape is easy because it's just below the E shape (same root notes) and C shape is just above the D shape. Pay attention not so much to the notes themselves (we're guitarists) as the intervals: the intervals will help you with your chord theory... as in helping you with what makes (in basic terms) a major, or minor, or non-specific (like power chord where major or minor isn't explicit) chord. After that you can start picking up why a Gmaj7 is called that, and what the devil an F/B#sus4 might be... once you get the hang of it you start getting "EUREKA" moments as something you knew about but didn't understand suddenly become clear to you.

Scales are fundamental to all music and are a pre-requisite for a decent guitarist. The reason most guitarists start out with chords is because you only have to learn 3 of them to do a CCR number. In fact, if you can play Dm you can pretty much handle the whole of Run Through The Jungle. I digress.

If you don't go for a decent book to help you out then check out:

http://www.ultimate-guitar.com/lessons/
http://www.seymourduncan.com/blog/

I'm sure there's millions more, and the Ultimate Guitar one some of the articles are poorly written in that the writer seems to know what he's talking about but explains it badly. The Seymour Duncan Blog has some great "snippet" type lessons. Don't be alarmed by articles that have "Chord Substitutions" or "Inversions" in the titles: invariably they'll dip into scale theory to explain why it is the way it is, and that's always good for background knowledge... even if you don't understand it the first time, if you stick with it then one day you will, and you'll have one of those "EUREKA" moments that enriches your life forever.

It's worth it, is what I'm saying.

Honestly this is the only book worth getting... unfortunately it's not in paperback form.

http://www.amazon.com/Fretboard-Theory-Desi-Serna-ebook/dp/B004ASNACS/ref=sr_1_1?ie=UTF8&qid=1386173281&sr=8-1&keywords=the+fretboard+theory

To all future readers of this thread... it is only going to be as hard as you want it to be. Learn a scale by all means, relate it to a key but, from that point onwards, use your ear as you practice that scale against a chord progression of the same key. A book is not going to tell you how to play it, only you can play it and use it.

Scale books tend to overdo stuff and just flood you with several fretboard diagrams that mean nothing. Learn the notes on your fretboard and you're ready to tackle any scale formula without the need for a diagram. Chord books tend to do the same thing as they don't tell you how to use it or where (the only amendment to this is Ted Greene's Chord Chemistry - I dare you to read it and apply it. You can thank me later).

Robben Ford's one video describes the scales he uses over certain chords, and he even tells you why (I say, the nerve!!). It is in book form too so get searching. However, knowing the scale alone is not going to help you find the magic note. You have to play with it, despite what several parents/people say, to know what sounds good where and what is just a carrier.

Carol Kaye ( session bassist from 50s to today, you've heard her playing) emphasizes chord tones rather than scales per se- so rather than learning scales, understand what notes go with each particular chord, which depends on the chords tonality and the key of the song, and even what the other instruments are doing. There is some stuff here that helped me to understand this approach. Bear in mind that she's talking about bass so technical things like 1-2-4 fingering are not really applicable to guitar but the theory stands.

peterleroux wrote: Carol Kaye ( session bassist from 50s to today, you've heard her playing) emphasizes chord tones rather than scales per se- so rather than learning scales, understand what notes go with each particular chord, which depends on the chords tonality and the key of the song, and even what the other instruments are doing. There is some stuff here that helped me to understand this approach. Bear in mind that she's talking about bass so technical things like 1-2-4 fingering are not really applicable to guitar but the theory stands.

As most bass players I started of on a 6 string classical guitar. Studied that for a year or two and was given an opportunity to play bass(more like forced) So I was given a very cheap bass guitar by the then band leader(it was a FP look alike and it had plastic coated strings on) He told if you can play the chords then you play the bass he said. So I had a folder full of songs including all the guitar chords and of I went.

What Kaye is referring to is a good method to learn bass as well. If you understand chord structures and you know which chords are used in a song one can easily come up with a very good bass line or even copy-perform a cover of the original bassline by simply listening to the song.
With experience you learn how to link the "chords" i.o.w. to get from e.g. C to C7 and then G.
Know the difference between a D and a Dm bass line etc.

@peterleroux, thanks for the Carol Kaye Link. I have just started playing bass again and this is wonderful reminder material. Thanked

Hi Nitebob, here is a very quick answer, if you want it: looking at the notes in the G major pentatonic scale, you'll see that they are in fact the same notes as found in the C Major scale. So you have not changed key...just a change of orientation towards a note in the scale....I will try a even clearer way of saying it.....

Notes in C Major: C; D; E; F; G; A; B
Notes in C pentatonic: C; D; E; G; A
Notes in G Pentatonic: G; A; B; D; E

If enough of the piece supports the C Major key, the G major pentatonic will not throw the key off.

To answer this:

I ended up spending far too long thinking about WHY scales are they way they are, without ever working it out, until I realised that they just ARE and I should just get on with learning them.

There is some very good info on this here: http://www.musicmasterworks.com/WhereMathMeetsMusic.html
and here: http://www.music-cog.ohio-state.edu/Music829B/main.theories.html

It's a very neat way of seeing the scale...as it says in the article...annoyingly inconsistent at times, but mostly it holds true.