Prostitution is a way to earn money by selling sexual services. In The Sims, it's also a great way to master those pesky "Have XX friends" Charisma challenges quickly.
To start prostituting or 'whoring' themselves, a sim needs a minimum of level 1 in the Woohoo skill.
At the start of a prostitution career, the sim's reputation is low, and the police may harrass them if they solicit in public locations.
Most sims start prostituting themselves in Dive Bars, and work their way up to higher-class establishments.
The game will send a handful (three to six?) NPCs to the lot. You can either wait for one of them to ask, or take the initiative via (Target sim) -> Kinky -> Whore -> Solicit. Upon successful solicitation, the game will display a price list, and the sims will queue up a Woohoo job
Note: Soliciting sims who live in a lot in town does not seem to work very well, particularly if you've never talked to the target before, and early in the game when nobody's horny yet. I found it best to focus on the sims that arrive shortly after you start whoring, even if that means doing one of them several times.)
Note: Selling an action for the very first time seems to give a cash bonus (my sim earned 1500 Simbucks for her anal defloration IIRC).
Note: The home base for tourist sims in France can make an excellent location to start whoring due to its low risk (no police interference), furniture (no tents like in Egypt) and central location (unlike China).
After making enough money from various sims (Woohoo skill of 6 and $8000 of earnings), you'll get the option to enroll in the Callgirl service.
Callgirls can go to a Hotel where they will serve their client for six hours. They may also be asked to visit a resident family's lot. The main client may ask them to woohoo with another sim present on the lot.
The Escort is the top of the rung of this "mini career". You'll accompany VIPs to Cocktail Bars and Dance Clubs registered in KW (I guess, my sim isn't quite that far yet.)
Hotel: To set up a hotel, I recommend setting a Hangout lot type (these are open 24 hours). Place beds, a Bartender bar, and maybe a bathroom and some more furniture, then register the building with KinkyWorld by clicking on its walls.
Cocktail Bar / Dance Club: KinkyWold has an option to register these, I guess they're for Escorts, but the last time I played a sim for that long was around Build 275 ...