Nothing too hard, good for beginers, and once you master it there are many variations

Its really just one motion in 3 different places along the bass. 

G|————————— |—————————| 
D|———————4— |—7—4—————| X2
A|———4—7——— |—————7—4—| 
E|—5——————— |—————————| 

G|———————4— |—7——————|
D|———4—7——— |——4—7———| X1 
A|—5——————— |——————4—|
E|————————— |————————|

(repeat first 2 bars)

G|————————— |—————————| 
D|———————4— |—7—4—————| X1
A|———4—7——— |—————7—4—| 
E|—5——————— |—————————|

G|———————6—| 
D|———6—9———| X1
A|—7———————| 
E|—————————|

(repeat of the same motion 2 frets over)

G|———————4—|
D|———4—7———| X1
A|—5———————| 
E|—————————|

(repeat first 2 bars again)

G|————————— |—————————| 
D|———————4— |—7—4—————| X1
A|———4—7——— |—————7—4—| 
E|—5——————— |—————————|

There are many variations for this 12 bar riff. one variation is to play the same peice 
only backwards. it still has the same base notes, and will work to the same song, only 
sounds a little different. you could also change the first note in the second bar to the 
5th fret, which would look like this: 
G|————————— |—————————| which sounds a little different, but once again, would still 
D|———————4— |—5—4—————| work in a blues scale. another thing that you can change is the 
A|———4—7——— |—————7—4—| last 2 bars to make a different closing to the scale. there are 
E|—5——————— |—————————| a lot of different ones, but this is my opinion is the easiest:
(play the notes as doubles, not individual notes)
G|———————————————————|———————| this is to replace that 11th and 12th bars only. when you 
D|———————————————————|———————| play it, pause after the first bar, and play the last 3
A|—————4—4—5—5—6—6—7—|—5—6—7—| notes quickly. and now you are a masta of the 12 bar blues
E|—5—5———————————————|———————| scale.