Steve has to stand on top of a solid block, so if he's
standing ON level 0, then his knees are on level 1 and head is
on level 2. When mining, I personally find it easiest to refer
to knee level. So if you want to mine blocks that appear at
level 13, you'd want Steve's knees on level 13 (standing on
top of level 12, and head at level 14, and typically mining
levels 13 & 14 together).
Where is bedrock? GamePedia claims it can be found up to level 9, but I have only found it in levels 0-4, just like the PC version. Level 0 is 100% bedrock. Only the top part of the block is rendered, so if you remove it (in creative mode), you'll immediately see the void and NOT the sides of neighboring blocks. Levels 1-4 are partial bedrock. So if you dig down and start to find bedrock blocks, take note of the highest level that you see bedrock, and call that level 4, and then you'll know where to mine for items like diamond ore (levels 1-15).
Where is lava? All sources seem to agree that MOST lava lakes crest at level 10. This is exactly what I found doing a test at a typical lava lake, as shown in the picture below. Levels 11 & 12 are the most popular to mine for diamonds, so one strategy I use to look for diamonds is to find a deep cave, find a lava pool (not lava flow) in the cave, look for exposed diamond ore around it, and if there isn't any, start digging a horizontal tunnel very close to the lava through levels 11 & 12. (NOTE: On rare occasion lava lakes will spawn at other levels. If you don't find diamonds, confirm the correct depth by looking for redstone ore OR dig down to bedrock. But almost all the time the lakes are on level 10.)