An aquifer simply is a layer of wet rock that keeps making more water. It's a bit of a challenge to dig under one, so you should avoid aquifers unless you really want to try out building pumps and stuff.
If you want to see what different places (aquifers, evil ocean beaches, whatever) are like, use the location finder before embarking. Some places are like a dwarven paradise ... others will wipe you out by the second year.
(I also use the Finder to avoid aquifers, rather than modding the game raws ... same result though.)
The size of your starting location equals the size of your "level" later, where you build the fort. 6x6 is a fairly large area, some players (possibly those with slower computers) prefer like 4x4 or even smaller. There's no way to get past the borders of this area in Dwarf Mode (not yet).
After you've activated dwarves into the military, press m to get the military screen, and w to see what weapons and armor they're using. I'd recommend chain mail and wrestling at first, though you'll want a crossbowdwarf or two soon.
On the (m) military menu, scroll to a soldier and press v. Now press s, a and t (s = "sleep in room" to keep him happier, a = squad harrasses dangerous animals, and t = off duty. Soldiers will train their skills while off duty, which is a good idea.)
It's also helpful to put an armor stand or weapon rack near your entrance and assign it as a barracks, so if a Kobold Thief turns up there's usually a sparring military dwarf nearby.