Cutter John wrote:
I seem to recall reading that iron only occurs 10 blocks below sea level or deeper. But yeah, mineshafting is the way to go.
Iron ore can exist 5 blocks above sea-level, but it's rare. According to the wiki, here's the levels where all the various ores can be found.
Coal: All levels
Iron: Up to level 67 (5 blocks above sea-level)
Gold: Up to level 35
Lapis Lazuli: Up to level 32, most common at level 19
Redstone: Up to level 18
Diamond: Up to level 18
Lava pools: Up to level 10, can very rarely be found on the surface.
If you're looking specifically for diamond, mining between levels 10 and 18 is safest because you wont risk lava pouring in on you, all you have to worry about is accidentally mining into a cave filled with creepers.
Mining obsidian is painfully slow, it takes 15 seconds for each block. If you really want to enter the Nether and don't want to go through the process of mining for diamonds and then mining all that obsidian, you can make a portal-shaped cast with dirt, use a bucket to fill it with lava and then pour water on it until you have an obsidian portal.