You can't really build houses per se, you live in one house that you choose to place down at the beginning of the game. And that house is the one you can customize and expand. The town works projects involve things like bridges between rivers, lampposts, new fountains, fences, etc. They are projects that have a set amount of currency that needs to be met to have them get built. You're essentially customizing your town. Think of the projects as glorified furniture you place down on the map. (Of course they aren't *literally* furniture). You can certainly create your own garden, though, using the flowers that you can buy in the game.
In Animal Crossing, you have to make money by either selling items, finding the money in the town via certain tricks/secrets, or you can take up a part-time job (which is something that's new in ACNL) working for Brewster at his coffee shop. So a big part of the game is having a large sum of money to do what you want, as that's also how you buy clothes and accessories to customize your character and buy things to customize your house (this also includes furniture).
Granted, you don't *have* to solely focus on making money, but it's one of the important things you will need to manage.
It's not like the Sims where you're managing a family, you move in with money already in your pocket, and command your character what to do. You actually CONTROL and move your character around the town like any old 3D game. You're literally moving the character with the analogue stick and everything. You start from 0 and go all the way up to 10, you have to work your way up and along with that comes events, sharing items between friends via online, talking to neighbors (possibly doing chores for them), collecting bugs/fish/fossils, and so much more.
