40d:Location

Starting out in the right location is crucial to not losing (but remember, losing is fun!). Beginning players have several things to keep in mind when selecting a site.

Surroundings

It's probably a good idea to avoid Haunted, Sinister, and Terrifying biomes, as well as extremes of cold and heat. Fortunately that still leaves you with a lot of options most of the time. Make sure you've got at least some trees and vegetation.

Make sure you at least have contact with Dwarves; Humans are also good trading partners. If you get those two, you'll probably be around Elves and Goblins too. Elves will trade some things, but are picky about what they'll accept; Goblins will just lay siege to your fortress every so often once they get angry enough about your presence.

Terrain

Since you can dig down, you don't have to worry about finding a suitable cliff face (although players of the previous versions might find it reassuringly familiar). Still, mountainous areas are worth looking into for the stone and greater probability of finding magma.

Water

Farming won't get you much in the middle of a desert. Try to find an area with a brook -- larger water sources can hinder mining. If the game warns you that you've selected an area with an aquifer, pay attention: it's likely going to be very difficult to get through it to the stone below.

Currently a permanent source of water isn't required because farms don't dry out, this is expected to change. If your map starts with even the smallest pond you can dig under it, drain it into the room (and down again if there's that much water), and build a farm on the residual mud--water levels of 1/7 can be ignored when placing the farm plot.

Lumber

Not as important in the new version, since caravans now bring wood; but treeless maps should be avoided by new players.

Trees grow on the lower surface z-levels, so make sure you have a nice large swath to chop down. Just because the biome says "heavily forested" doesn't mean you will actually have trees, and just because the biome says "none" doesn't mean you won't have trees. (last sentence added by Savok and from memory; possibly wrong)

Vegetation

The local flora can be a good source of seeds.

Magma

Unlike in previous versions, you aren't guaranteed to find a source of magma, unless you have a volcano at your starting location. Sure, you could burn charcoal to fuel your smithies, but the convenience of magma makes it invaluable. The site selection screen can give you a good idea of whether or not you'll be able to get any: look for darker igneous rocks like basalt, obsidian, gabbro, and so on.

Towns

Towns created by other civilizations exist only for your benefit. Humans won't mind at all if you tear apart their main pub to build your tunnel entrance. Also their buildings provide plentiful wood and other useful items such as prebuilt furniture.