So if you've been playing in one basic area for a rather long time and you have depleted all your ore resources, well, this is for you!
Step one: Preparing for the inevitable
The first item you need to survive is wood, but this is for experienced users who have depleted the area around them of most of their resources. You first need to make sure you have the following items:
- 1 or 2 iron buckets
- at least one sapling of some type (except dark oak saplings, in which case you need 4)
- access to at least one source lava & water
- melon or wheat seeds
- a farm of some type
Now the first thing you need to do is create a 'safe zone' (i.e., somewhere lava cannot set things on fire) and a tree farm. With your iron buckets place lava on one side of a hole around 3 or 5 meter wide, now on the opposite side place water this will create cobblestone which can be used to make axes or hoes. A tree farm should be set up so as to gain maximum yield harvest so place your saplings depending on your tree type around 3–5 meters apart. You now have a rather steady source of wood for repairs and pickaxes or other needful things. Now (assuming you are not on peaceful) you need to have a farm of some sort. If this is a wheat farm the best way to go about doing this is to make an infinite water supply.
If it is a wheat farm it should be set up as large as you care to take it so as to have max yield. If it is a melon farm the following should be used:
With these you should be living indefinitely. Good luck.
For additional common resources, such as iron, special advanced farms can be built. An iron golem farm is the best way to go pre-1.8. For that, you need to gather at least 2 villagers (using minecarts or similar tricks) and breed them using houses (a door with a block above counts as a house).
With 1.8, villager breeding has been changed, so now you need to trade with at least two of them to make them "willing". This can be considerably harder at times, but that's your problem.
The best strategy to get easy villagers on maps without villages (such as Skyblock) is to get a blaze rod from the nether (in Skyblock, this is the hardest part, as you need to find the place where a nether fortress used to be, either using mods or by randomly building spawn platforms throughout the nether, until fortress mobs spawn). You then craft a brewing stand and brew a splash potion of weakness to throw at the next villager zombie you meet. You should capture him before, of course. (In Skyblock, glass bottles can be gathered by killing Witches that spawn occasionally). Then you only need to feed him a golden apple and wait (in Skyblock, gold can be looted from Zombie Pigmen).
Do this twice, and you are ready to go - infinite iron supply!
(Again for Skyblock, don't screw it up. Sand is not renewable, and Witches (which may drop glass bottles) spawn infrequently. You only have a very limited supply of glass to use for your potions!)