The Sims, in all its versions, adhere to some parts of real life. One of those parts is obtaining some form of work for your Sim in which to get a steady flow of income. As with real life, this income goes to pay for all sorts of things, like bills, recreational activities, and stuff for the home. There are two ways of getting simoleans - the first way is the way that you or I make money in the real world.
By working for it. You find your Sim a job somewhere, either a part time job or something that will lead to a career, and they go to work on their work days and get paid a daily amount of simoleans.
While this is the more proper and 'real' way to go through life with your Sim, there is of course the much easier way, which is the Sims 3 money cheat code. To be fair, there are actually two money cheat codes - one that gives a small amount of money (kaching) and another that gives a much greater amount (motherlode).

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Kaching will add 1,000 simoleans to your current Sims income. This is good in the event that you are short on funds for something, like that smoke alarm for the kitchen.
Motherlode adds 50,000 simoleans to the current Sim income, making your Sim a virtual millionaire. This is mostly used for families, allowing for the user to add funds in order to buy essential items (like a crib for a new baby) or for moving into a bigger house.
Both of these cheats need to be entered in the cheat window, by pressing CTRL+SHIFT+ALT+C at the same time. These cheats can be used from any screen where the current household is available.
Image credit - Sims 3 logo, SIms 3 game box @ The Sims 3