You have built a house or bought it without furniture - it's time to furnish it.
Go to buying mode (F2). Here you can find many objects: drawers, pictures, TV set and kitchen appliances, mirrors, beds, flowers etc. You can categorize object according to rooms: kitchen, bathroom, bedroom, living room, dining room, study, children's room, garden . Folds at the top present different rooms. Simple system of icons enables you easy access to useful objects.
Objects can be also categorized according to their usage : fittings, appliances, surfaces, comfort, electronics, entertainment, lights, decoration, storage, for children, vehicles. Entering each category you will find folds according to the type of the object.
You decide which one is the most comfortable for you.
Every object has information on the influence on different Sims' needs and how fast. Fir example bed influences energy  - Sim who sleeps in it regenerates energy. The higher the number at the info about energy, the faster Sims gets enough sleep. Situation is identical in the case of other objects. Moreover, some objects enable developing skills, for example mirror enables studying, charisma and chess or telescope  logic. Objects can be situated as you want, even slantwise - you turn them by using keys "," and "." (comma and full stop).
The heart of every house is kitchen - the same situation is in The Sims 3. You definitely need fridge and few standing table tops. Don't forget about cooker and about fire alarm near it . You don't have much money _ buy the cheapest or almost the cheapest things. Don't save money on dishwasher and trash crusher - buy them at once and they will make your life easier. It is sensible to ingredients preparator and if it's possible microwave. Everything set in such way that there is one table top between fridge and the cooker where Sims can prepare meals. Add lights, on the wall or on the ceiling .
At the beginning you cannot afford to build a separate room for dinning room, so put table with chairs, so Sims don't have to go far and waste time on going between dining room, and kitchen . Use style creator to modify the outlook of all objects (as in designing clothes for Sim). In the future you will make kitchen bigger so it will be more comfortable, especially when there're more Sims in the house .
Bed is a very important object - the more energy points it has the faster Sim will regenerate his or her strength. For now you can buy a single bed, if you want to have someone for your Sim you will need a bigger one, double bed. Perfectly if it has at least 5 energy points. Pay attention to proper light  - unless you have much money, it is not end of your expenditure. In bedroom put chest of drawer (click on it to change Sim's appearance) Put small table next to the bed, and alarm clock and lamp on it . If you want you can put on hang mirror.
In the future you can make bedroom bigger so that a big bed which make Sims regenerate even faster .
It is a very important room. There must be a toilet (buy the cheapest one, it gives +10 to the bladder anyway) and shower and bathtub. You can also put basin or mirror. Don't forget about some cheap light . Remember about locating the bathroom itself - it should be easily available for example from bedroom and kitchen. In the future you can think about more rich furnishing, even if you make the house bigger, it should be accessible . When there are many Sims, you can make a separate toilet and bathroom or just make another one with all useful appliances.
You can't afford to buy too much. You really need a desk with chair and computer. If you can afford it or you just feel like it buy a TV and something to sit in from of it: sofa or chair . You should also buy a bookcase (if you have some money of course).
In the future, when you can afford it , decorate living room with a real flow  with armchair, sofa, decent TV, stereo, stylish lamps, sculptures or pictures etc.
You have to have two more objects: phone (somewhere near living room or kitchen) and - it is necessary (!) - burglar alarm at the door .
In the future think about building additional room where you can keep objects like running track, chess, easel, which are necessary for Sims to develop skills . The best option is to settle it on the ground floor. There are (at least for now) no elevators and Sims just go very slowing on stairs which is a waste of time.
If you have children, they should have a separate room. Even a toddler should have his or her own crib in a separate room. Thanks to it the cry won't wake up the parents and we decide when to go to the child. Then we will change crib for bed, and when child goes to school, toys as well as a desk should be in his room .
It is sensible to make a side  or back exit (for example from kitchen), so Sims can reach the swimming-pool, garden, playground or other places situated outside the house faster. Later you can think about the interior design. Instead of buying better appliances, hang pictures and curtains, put sculptures and flower  - it influences Sims' mood well and is more pleasant for the player.