@DanielB posted a list of Youtube backing tracks for people to jam with.  Did you know that the vast majority of rock, blues and country music follows the simple 1-4-5 chord progression? That's true for both major and minor chord progressions, but let's look at just major chord progressions for starters. If the song is in the key of G, that means that the chords are going to be G(root)-C(4th)-D(5th).  Pick any key, and the formula is the same.

There is a musical scale that will allow you to never play a bad sounding note with any of the chords in a 1-4-5 major chord progression.  It's called the Major Pentatonic Scale.  Since Penta means 5, it means that the scale has only five different notes in it.  They are: root,2,3,5,6, then the next note is the octave. So, by simply dropping the 4th and the 7th notes from the scale we can play all of those notes and never hit one that will sound sour.

Let's see how that would look for playing in the key of G in first position. We'll be playing G-A-B-D-E-G and the fingerings will be:

G:  0-1-2

D: 0-1-3

A: 0-1-3

E: 0-2

To play any major pentatonic scale using a shifting pattern (no open strings), one would always start at the root note and proceed from there.  The pattern for scales starting on the G string is:

G: 1-2-3

D: 1-2-4

A: 1-2-4

E: 1-3

So just slide your index finger to the appropriate root note for the key and play that pattern and no note you play will sound wrong.  Rock on.