I always have the worst time finding Diamond. I saw this one guy pull up this information which told him he was at the right level to find Diamond, but I never figured out how to do that.
Are there places like mines or whatnot that have diamond, or is it really just anybody's guess?
Dig down. Diamonds are at the lowest levels.
Highest chance to find diamonds are at y12
All other answers are just totally incorrect…
Diamonds will randomly generate in the walls from y=0 to y=16. Lava generation is at y=11. Diamond generation does not get more frequent if you move further up, or down.
When you are beginning, the fastest way to acquire diamonds is to dig a hole down to y=13, then dig a 1x1x2 tunnel for as many blocks as you want, return to the start, move 5 blocks to the side, and repeat the process.
Once you've killed a wither and got a beacon, it's fastest to get eff V tools, set up a haste II beacon, and just completely destroy everything at y=12 and y=6.