Let me attempt an answer.

I have to assume you meant "how do I find the chords of a song by hear?" Of course you could go online and get the tabs. You'd play the song but would not learn anything.

Learning a song by hear is not all that easy. It's takes a lot of practice (I guess you won't mind practicing a lot since your navigating on PB's site).

1. The more chords you know the better. Try to learn all the basic major and minor chords for starters.
2. Try to play over the song only on the 6th and/or 5th string. Once you can do that, you'll know the base note of each chord.
3. Once you have the base note of the chords, try to play chords from the base note. Say you find that you can fit an A on the 5th fret of the 6th string, try to play an A major barre chord, Am, A5, A7... and see what fits best.
4. Write down what you find.
5. Compare yourself with a few online tabs (in case the first one you get is bad).
6. Start with a super easy song (you can hear it has only a few chords)
7. Don't get discouraged if you don't get any at first.

Let us know how it goes.