Tutorials/Things not to do

Most tutorials are about what to do. This is about what not to do.

Sorted (loosely) by ascending order of obviousness.

Don't Dig Straight Down
This one is fairly straightforward. If you mine the block you're standing on (and don't know what's under you), you're quite likely to fall straight into a cavern (and die/be injured from fall damage), fall straight into a lake (and drown), fall straight into Lava (and burn), or fall into a Dungeon and be mobbed by monsters.

Digging straight down kills. Don't do it.

Don't Dig Straight Up
This isn't quite so obvious. However, if you mine the block ahead of you, you can release all sorts of nasty things directly on top of you. Gravel, Sand, water, lava... it's not fun to suffocate, drown, or burn. Again, don't do it.

Don't Try To Kill Creepers On Your First Night
Creepers aren't quite like other monsters. Just read their Minepedia articl- HISSSSSSSSS- BOOM

Just don't try it until you have a Bow.

Don't Mine That Valuable Block
If you don't have a Iron or Diamond pickaxe, many ore blocks (Redstone, Diamond, etc.) won't drop their valuable contents. Instead, they'll just be destroyed.

Wait until you have a decent pickaxe before mining valuable blocks.