basically if i song is played in the key of e then you will use a scale that will fit in it an e pentatonic for example is the easiest.
the pentatonic scale for that e is:
-0--3-
-0--3-
-0--2-
-0--2-
-0--2-
-0--3-

so if you play any of those notes whilst soloing over a song in the key of e it would fit. you can tell the route note in the scale because its the deepest i.e. the open e on the bottom string. if you were to take that pattern up to fret 5 for example it would now be in the key of a