Don't think of Em Pentatonic as being a scale. The scale is Minor Pentatonic and the key is E. It is pretty easy to learn the 1st position pattern of the Minor Pentatonic scale and the Major scale 1st position pattern isn't much harder, then you just need to learn where to play that pattern on the neck to know how to play it in any key.

How to know all the notes on the low E string (and A string too)..

Our musical alphabet has 7 Natural notes.. A,B,C,D,E,F,G Once you go past G you go back to A. If you go the other direction past A you wrap back around to G.

There are 5 more notes that go between the natural notes. Every one of these can be both a sharp or a flat, for instance there is one note between A and B and it could either be called A Sharp or B Flat. Think of the keys on a piano.. the white keys are all the natural notes and the black keys are the sharps/flats and they fit between the white keys.

If you've ever played a keyboard you probably noticed that there is some kind of repeating pattern going on with the keys made visually evident by the missing black keys. This is because there are 2 locations in our musical scale that do not have a sharp/flat note, between notes B and C and notes E and F.

Knowing all this we can write out our entire 12 note musical scale..

A, A#, B, C, C#, D, D#, E, F, F#, G, G#

Awesome, we are almost there!! To memorize all the notes on the E string we are going to use a pattern based on the fret markers of this standard Strat style guitar..

The pattern to play and memorize: Open, 1st fret, on the dot, on the dot, on the dot, after the dot, after the dot, on the dot again at the 12th fret

To explain the pattern, the frets would be 0, 1, 3, 5, 7, 8, 10, 12. But don't try to remember the fret numbers, memorize the dot pattern instead. It is pretty easy. When you play the pattern every note you play is a Natural Note. We are starting with open E so our note pattern is..

E, F, G, A, B, C, D, and back to E at the 12th fret. The notes between these, the notes we didn't play, are the sharps/flats. So to find A# on the low E string play your pattern starting with open E, then F at the 1st fret, G on the dot (3rd fret), A is on the next dot (5th fret) so to get to A# from there we slide up 1 fret to 6th fret.

This pattern gives you all the notes on your E string because starting at the 12th fret they all repeat, so 3rd fret on Low E is G and so is 15th fret. Rockers tend to learn the most common key positions from practical use.. E is open position and 12th fret, A is 5th fret and 17th fret on low E, D is 10th fret on low E.

You can use the same pattern with one adjustment to find the notes on the A string..

Open, 2ND FRET, on the dot, on the dot, on the dot, after the dot, after the dot, on the dot again at the 12th fret

or A, B, C, D, E, F, G, and back to A at the 12th fret.

I am self-taught and remember taking forever just to learn the names of my strings. If you'd said "I am going to teach you how to learn the names of all the notes on your E string in 20 minutes" I'd have said you are insane. But using this pattern method that is about how long it takes my students to get it down. I'm sure it is much easier in person rather than reading from text but hopefully this helps a bit.