Consider I have a house:
A Sims house. With one of the new carpets available in Nightlife.
To build a garage, you will need a driveway. In the real world, people can (and often do) drive over grass or muddy terrain to get somewhere. Sims aren't like that. You need at least a driveway before you can have a car.
Driveways live under the Build Mode section, next to the stairs. Click there and you have an option as to whether you want a concrete one or a brick one (the latter being 50 simoleans more expensive).
Placing the driveway. Must connect with the street or you can't place it.
When you place a driveway, you must have it connecting to the street as shown. Otherwise you don't have a driveway, you have a lump of concrete. Because of the size of the driveways, you may want to consider adding them to new lots, as it may prove hard if not impossible to add it to existing lots without a lot of fiddling about.
You can place two driveways next to each other as well but they can't overlap. There will also always be a backend requirement to allow the cars to turn in the street. Hence it's a good idea to put them on the LEFT hand side of the house, or build them first and the house around it.
Okay, you then need an extension piece which can only sit on top of the driveway you just placed. Then you need an up-and-over door.
Up and over door. Goes between extension and driveway only.
The up and over door can only go in one place - on the gridline between the end of the driveway and the beginning of the extension piece. Look for the green acceptance arrows for when it can go down. According to Maxis this is the hard bit. It is anything but.