1. Identify the key of a song. Each song is set in a key. Some are c. Some others are c#.. or d or d# .. and so on.
If you can figure out the key its easy to figure out chords. For example if a song is based on c, you can be sure thar chords c and g will suit that song.
2. If you have a keyboard, you can transpose it and try to find out chords. Either way knowing the scale or key of a song is important.
3. To figure out the key of a song you can try this. Pay the song and hold sa pa sa. If it matches with that sruthi or pitch, then that's the key of the song. (C or sa in this case)
If that doesn't match try c# g# and c# combination. You have to continue until you find out the perfect match for that song.
Also keep in mind that, chords are nothing but notes played in synchronous way. The actual notes from the song can be used as chord generating notes
Keep trying and you will find your way out. Good luck.