Originally Posted by: UllR
So I do not understand how to know what key a song is in.[/quote]
Short answer: pitch matching & practice. :)

Long answer! In order to listen to a song & figure out the key you need to listen for the melody & the chord changes.

You can figure out the melody by singing along with the vocal part or the guitar riffs or lines. Then find those notes on the guitar. This is called pitch matching. You match the notes you hear in the song to the ones you find on the guitar. Obviously, you will have to know your fretboard in order to name those notes! And you will have to know your basic major & minor scales to know which patterns the notes form to know what scale you are using.

The notes of a melody or riff will be all or mostly contained in one particular scale (major or minor). That's what key the song is in!

Another approach is to listen to the bass motion. This can be the bass line, or the lowest note in the chord as the chords change. Often as beginner guitarists we can figure out key signature by listening to the song one chord at a time & trying to play a low bass string note to find which one matches with the low chord note we hear.

This is also called pitch matching & it takes a lot or a little practice depending upon your skill level. People have different amounts of ability or musical backgrounds so this skill is easier for some & harder for others.

The good news is anyone can learn & get good at it. :)

Lisa's tutorial on chord theory can help with this.

https://www.guitartricks.com/tutorial.php?input=997

My tutorials on how scales & chords are related can help you get used to the sound & motions required to hear bass motion & chord changes.

www.guitartricks.com/tutorial.php?input=426
www.guitartricks.com/tutorial.php?input=427
www.guitartricks.com/tutorial.php?input=428

Originally Posted by: UllR
I do kind of understand the 1, 4, 5 thing, but how do you know when you listen to a song what key it is in.[/quote]
If you can find out from pitch matching which chord is the root, that is the I. Other chords will be related to the I by their position in the scale/key from the root. That's how you'll know which on is the I, IV or V (or ii, iii, vi, etc.!)
[QUOTE=UllR]And a song may not always stay in that key, is that correct.

Yes, but there is always a primary key. And then it might change to another key temporarily (modulate) or permanently (transposition). First work on pitch matching & figuring out the primary key, then you can work on secondary keys.
[QUOTE=UllR]
Also how does the Capo thing play a part. I know all this info is probably in the lessons but I can't find it.

The capo is covered in various lessons in the course material.

https://www.guitartricks.com/lesson.php?input=12807&s_id=676
https://www.guitartricks.com/lesson.php?input=12808&s_id=717
https://www.guitartricks.com/lesson.php?input=13396&s_id=767
https://www.guitartricks.com/lesson.php?input=14654&s_id=987

Essentially, the capo is a way of using open chord shapes in different keys. There are also other specialized uses!

Also, Caren uses it in quite a few songs & gives some great tips about using them! For example:

https://www.guitartricks.com/tutorial.php?input=1916

Hope this helps!