Originally Posted by: michael0609My question is how do we know wich A. C. E note to play?

Anywhere on the guitar you can find them, get your fingers on them & get them to sound. :)

This shape is the first A minor chord that many beginners learn.

E |--0--|
B |--1--|
G |--2--|
D |--2--|
A |--0--|
E |-----|

And it's usually first, because it easy to remember & play. But, it's also first because it's one of the most convenient ways to play the notes A, C & E.

E ||---|---|
||-C-|---|
||---|-A-|
||---|-E-|
A ||---|---|
||---|---|

This is also why we show the simple A minor chord in GF1 as this:

E ||---|---|
||-C-|---|
||---|-A-|
||---|---|
||---|---|
||---|---|

Make sense? All you need are those 3 notes anywhere you can find them. Later on you will find that since the notes A, C & E are located many places on the guitar, there are many places to play an A minor chord. :) Eventually, after you learn many possible places to play any given chord, it becomes a matter of which voicing (or way to play a chord) works best for the song you are playing or the sound you are trying to create.

I discuss this topic in depth in this tutorial on chord triads & inversions.

www.guitartricks.com/tutorial.php?input=148

But it is also covered in GF2, which is the best place for beginners to start.

www.guitartricks.com/course.php?input=2

Hope this helps. Ask more if necessary. Have fun!