12 bar Blues is just a cycle of a riff that can be placed anywhere on the fretboard and
you can determine where you play by knowing the root note of the first chord. This is
because 12 bar blues is based around this
root, root, 4th, root, 5th, 4th, root, this is repeated over again, a good way to
practice finger placement is to get a couple of friends (or a drum machine) and to
improvise over this structure. if you're wondering how I came up with this structure 
it's taken from the major scale.

G|---------------2--4--5|
D|------2--3--5---------|
A|3--5------------------|
E|----------------------|
  1  2  3  4  5  6  7  8

The root note is the first note played, it is where the rest of the notes are derived,
this is the Cmajor scale and the blues riff from this would be

G|-----------|
D|---5--5----|
A|3----------|
E|-----------|

you can move the major scale anywhere on the fretboard and use the notes to find out where to play.

the main blues/jazz riff is just playing
1st, 3rd, 5th, 6th, 8th, 6th, 5th, 3rd

in G major the riff would be

G|--------|--------|--------|--------|------2-|5-2-----|--------|--------|
D|------2-|5-2-----|------2-|5-2-----|--2-5---|----5-2-|------2-|5-2-----|
A|--2-5---|----5-2-|--2-5---|----5-2-|3-------|--------|--2-5---|----5-2-|
E|3-------|--------|3-------|--------|--------|--------|3-------|--------|

G|------4-|------2-|--------|--------|
D|--4-7---|--2-5---|------2-|5-2-----|
A|5-------|3-------|--2-5---|----5-2-|
E|--------|--------|3-------|--------|

You can change this to wherever the root note is