Listen to the song and take note of a few things:
- The beat of the song (is it played in 4/4, 3/4, 6/8 etc.)
- The strumming pattern of the song (for some songs, the guitar is really clear. Listen to the rhythm guitar and see if you can get the strumming pattern. If the strumming pattern is not clear, or if the song uses a piano, you can choose your own strumming pattern, as long as it fits the beat of the song)
- The bass guitar (most of the time, the bass guitar will play the root notes of the song. If you know what notes the bass guitar is playing, you can know the basic chords for the song)
The song I will use is "With Or Without You" by U2. If you are my age or older, you should definitely know this song. If you are younger, I guess this song is worth listening to. Here's the video. Take a listen to it.
- The beat of the song - After listening to the song, you should be able to find out that it is a 4/4 song.
- The strumming pattern for the song - You will realise that this song is basically bass guitar driven, so you can come up with a strumming pattern that suits a 4/4 beat. In this case, I would just choose 8 downstrokes per 4 beats, and even mute the strings for a more driving/rock effect.
- The bass guitar - Take note of the bass guitar (it's really obvious in this song). Listen to these bass notes and find them out on the guitar. You will eventually figure out that these 4 notes are D A B G.
I don't really know how to explain about figuring out the key of the song. But if you follow the rule of thumb I mentioned above, you should be right most of the time. Anyway, for this song, it is played in the key of D (what do you know, it's the first note of the song).
So now that you know the song is played in the key of D, you can look at the family of chords that belong to the the D family, and you will find out that you need to play D, A, Bm, G. Take note that the B is now a Bm.
So you have the beat of the song, you've chosen the strumming pattern for the song, and you know the chords for the song. There you go, you have just figured out a song.
I hope I've managed to explain it quite simply, but if you have any further questions or need to clarify something, please feel free to let me know.