This is a skill that takes time to develop well, but one worth working on, once you get past the beginner stage.  I’m going to give you some pointers, but if you’re starting out, you won’t yet understand some of the things I talk about.  You will, in time.  See also Developing Your Ear.

If you’re new to this, even if you’ve been playing a while, you’re going to find it difficult at first.  Don’t get frustrated.  If you really want to learn the song, but can’t work it out, spend an hour and then look for an answer.  You’ll still have done a huge amount towards developing your ear.  Some of the YouTube guitar lessons have mistakes, so if a chord sounds wrong, it might be.  Have a go yourself, you may be able to hear where they’re wrong and either work out the chord for yourself or find another lesson which is right. 

Many rock songs have simple 5th chords/power chords and not full chords.  I remember as a teenager working out 'Smoke on the Water', thinking that the chords were full chords in G.  They’re actually only double stops - in this case pairs of notes played on two adjacent strings in the same fret.

I’ll describe the process in stages:
•    Familiarise yourself with the song
•    Find the key
•    Try some chords
•    Write out the song
•    Find some example lessons

Then
•    Learn the song by memory

1.    Familiarise yourself with the song
You should listen to the song many times, paying attention to the melody (the song), structure (verses, choruses, bridge etc) and harmony (movement of chords).

2.    Find the key
By listening to the song, you should be able to hear whether the harmony is easy to work out or not.  Each major key has a relative minor.  They share the same notes and chords, but have a different chord that sounds like 'home'.  The relative minor of C major is A minor.

Using scales
While listening to the song, slowly play a major scale over it.  If it sounds wrong, try another scale until you get one which sounds right.  As you develop, you’ll start to hear whether your first attempt is too low or too high.  Once you have one which sounds good, that is the key of the song.

Using chords
Listen out for the part of the song which sounds ‘home’.  The part which feels like the other parts are leading to it.  It might be the last chord in the song.  This will be the key of the song.

3.    Try some chords
Once you have the scale, you can try some of the chords in that scale.  The obvious first choice if the scale chord, or the I (one) chord – so, in G it’s G.  Other chords to try first will be the IV (four) chord and the V (five).  Perhaps also the vi (six  minor chord).  More on this on the Nashville Numbering System page.

4.    Write out the song
I like start by writing out the structure and bar lines, as I listen to the song, then filling in the chords as I work them out.  Here’s an example:


As you can see, it only has four chords:  the typical I, IV, V chords – which in D major are D, G and A – along with the vi minor – Bm.

I write out the chords a number of reasons:

• Seeing the structure helps me memorise the song

• I write it in a way that helps me to easily read the chords and follow the song - in four bar sections.  At any time I might gig with a number of different bands and have well over a hundred songs in my head.  If we decide to do a song I haven't played for a while, I only need to glance at my iPad and go through it quickly on my guitar or in my head.  I find this format helps me do that.  It’s mine, so use it or find one that works for you.

• Note, I always find and add the lyrics onto a second page.  No point being in a practice situation with a band and not knowing where in the song you are.
  

5.    Find some example lessons
If it’s a simple song, you might not need to do this, but I sometimes get things wrong, so I’ll find some examples on the internet and compare them with what I’ve done.  Hopefully I’ll end up with something that sounds good!

6.    Learn the song by memory
In my opinion, you can’t perform a song if you’re reading it from a chart, and if you’re reading it, you don’t really know it.  If you learn the song and don’t play it for a while, you’ll forget, but you’ll easily remember if you’ve learnt it from memory in the first place.  I find that if you simply play the song over and over again whilst following a chart you won’t remember, so the next page is: Memorising Songs.