Starter Bed-Sit - The Sims 3
Here is a video of the building process in action. You may get some ideas from seeing it visually. This movie shows the creation of the cheapest house you can construct. Sure, it looks like a prison, but this is just to get you going. Once your Sim gets a job they can work their way up the monetary scale and begin to expand, as the latter half of the video shows.
Building the cheapest house in a bed-sit layout.
This house cost just over 4,000 Simoleons to construct and furnish. Not bad really, and this will have a contented Sim for at least a while. You could knock some money off that if you don't mind making your Sim miserable for a while.
You can then extend the house, even adding a balcony, once your Sims save up enough Simoleons.