Need some help with scales and stuff

Hey i've been playin for a bout a month now,and i know the basic note names and such and the whole step and half step,and the major 3's and minor 3's and stuff,but i just can't get scales into my head like what does it mean to play a song in the key of g or e or something,does that mean it's the root of it or what,i really need help with all this stuff like the circle of fiths.

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

that kind of helps but what do you mean it will fit in it fit in what,e?

E minor or G major, if I remember right.