I'm a minecraft newbie and i'm thinking of building my first shaft mine but i'm wondering how many ladders i need cause i don't want to be stuck down there. Also how far down should i dig?
If you find yourself stuck at the bottom of the shaft looking up to the pinpoint of sky high above, get a load of blocks - cobble or dirt - look down and jump. As you jump keep your right mouse button pressed and you'll place a block under you. Keep thi sgoing and you'll build your way out.
PS - 1 ladder = 1 block high so 64 minus whatever your level is to get to sea level.
As a couple people have said roughly 64 ladders (1 full stack). Really depends on what level you are starting your shaft at. IF you hit F3 you will see text one the upper left side of the screen look for the y: coordinate. That will tell you how many ladders you will need at that layer. Not a bad idea though to bring more than 64, I'd rather have a few extra rather than not enough and get to the end of my laddars and still have a few or more layers to go down and have to go back top side and make more oar grab more. There are many methods of mineshafts some dig down two blocks at a time others dig 4x4, others dig a stair case down. I tend to prefer the straight down approach. Whatever you do though do not dig straight down as in repeatedly digging the block below you. That is the cardinal sin of mining. You do not want to dig straight down and break into a cavern and fall into a pool of lava.... not good. If your going to dig straight down at least dig down in a 2 x 1 shaft that way the block your digging isn't directly under you. Now technically you can dig straight down if you are building the ladder as you go down and are on the ladder holding shift while you dig and add a ladders to work your way down. Your second question is how far to dig down. Again using F3 will tell you what level you are at, otherwise you won't know till you hit bedrock (cant break bedrock). Bedrock occupies the final 5 layers of minecraft. so the top layer of bedrock is layer 5 the layer at your feet is 6, the layer at your eye level is 7, and so on. Diamonds can be found at level 16 and below, but are typically most prominent at level 10-12, but as others have pointed out level 10 is the lava level so you do want to be above that so you don't mine into lava or at least drastically reduce your chances of that. Also remember that certain ores like diamond, gold, redstone, etc require an iron pick or better. A stone pick will break the block but yield no diamond.
also, bring at least a stack or a stack and a half or more of torches. You will want to light up your shaft and the room you create at the bottom. My shaft has a 7 x 3 room at the bottom of the shaft with a double chest, a work bench, and a couple stoves for smelting. You'll want those three items or at least the resources to build them with you. You can store your loot, build things like more tools/weapons/armor. Bring plenty of wood so you can create lots of sticks for building toos/weapons and torches.
You want your shaft, mining room, and the actual tunnels you dig to be well lit. Too dark and you will find your not alone in your mineshaft and tunnels. Would suck to finally come across some diamond only to to have it blown to bits by a creeper who snuck up on you.