If you decide to buy an empty plot, the distance to the city centre and other service points is crucial . Pay attention to the size of the plot , but don't get crazy at the beginning - you need simoleons to build and decorate the house. Move into the house and stop the game (by using "~").
Move to the building mode (press F3 or the icon on a bottom panel).
On the left  there are: hand tool (it enables moving objects), create a style (enables changing the outlook of objects), dropper tools (it "sucks" the object with its style to the hiding-place and enables putting the same element somewhere else) and hammer tool (which destroys and tears down pointed elements). On the right you can see the images of particular elements of house building. Starting with foundations, walls, windows, doors etc. ending with swimming-pool, trees, flowers or the level of the ground. You just have to click on a proper icon to take care of the proper element.
The shape of the ground
After confirming the purchase, you will see your empty plot ,where there's at least a tree or bush as well as the inequality of the ground which will make it impossible to build a house. In building mode enter ground tool  and use the option of flatting the plot to make the ground as flat as the street.
However, if you want to create a house on hills or even on a mountain  the tools of making the ground higher or lower are used for this purpose. You can create a pond by using water tool l .
Foundations and walls
The house may be build on foundation or on the ground.
In the case of foundations  remember that it is an extra cost and the need of building stairs everywhere (to main entrance  and possible one at the back or at the end of the terrace), they make it more charming, though.
No matter if it's on the ground or foundations start building walls. You can built it one by one , or start building rooms right away . You may built it slantingly and set the whole house in such way (you can do the same thing to the objects).
The point is to plan the house correctly. You need bathroom, kitchen, bedroom and living room where you will put television, desk and other elements - it is minimum. It's good to have another room for additional staff such as table for chess, running track of other objects you need to develop yourself (it depends on the professional career you choose), but it is not necessary at the beginning.
Bathroom should be at least 3x3 of field and if it's possible to be easily available for example from bedroom or kitchen - it saves time. Bedroom has to be separate room. Kitchen and living room don't have to be separated - you can make one, big room. It all depends on your preferences. In the future you can move walls, rebuild rooms. For now do what you can afford .