Where do i find diamonds in minecraft?

Usually around or below level 16. Diamonds are RARE and randomly generated so in all honesty you could dig for hours and still never see 1 diamond. I once had a map I played on where in my mine iron was more rare than lapis lazuli; yet my friend had mined 2 full chests of it and his mine was half the size of mine. It all just depends on what your location is and what map. Goodluck!

Anywhere between levels 2-16.

Diamonds are very rare. I suggest building a staircase down to bedrock level, and digging around.
Just don't dig down.

Underground below the y level 16.

The most reliable layers are 9-12. Get your map out and go down until it says Y=9,10,11,12