1. Its experience + understanding of theory.
Let me give a small tutorial on how i find chords. I play the song on keyboard (just find the scale that its composed in). Once you know the scale, use the circle of fifths (for reasoning) and key chord chart (for reference) to see what chords lie in the same scale. Also make up your own chords from the scale and try them. Eventually you will be able to find the chords of the song that will sound good when you play along. Note: The original chords played by the composed might be different and its very hard to figure out the original chords by listening (experience comes here).
Here are great tutorial on how to figure out chords of any song on keyboard:
2 . dont know
3. Just get in the groove and punch the hell out of your guitar. You will eventually end up with a homemade strumming pattern. Also, make sure that you complete one full measure (eg: 4 beats on 4/4 time signature) before repeating the same pattern. If you cant switch between chords the keep strumming (even though you mess up the chords) and you will get it in few hours of practice ... its all about practice !
"There's something magical about music and the simple fact is that it's a vibration of air. It's sort of magical if you vibrate the air in a certain way, people are pleased by it." - Damien Rice