A sim doesn't HAVE to have a "real job". But your sim needs to earn money somehow if he/she is going to buy food and survive. That can be making paintings and selling them, or whatever the quest is. Often it works well to have one sim working while the other stays home doing family stuff :)

There are two main ways to find a job - read the paper or look online.

The Sims 2 Tips, Techniques and Walkthroughs