Choosing a place to live is one of the most important decisions a consumer makes. The three most important considerations are location, size, and price. Once you know the number of bedrooms you need, a rent range that you can afford, and the area you want to live, you can look for apartments through various sources.
Rental listings are published on many websites.
Large national sites like rent.com and apartments.com have listings all over the US. Property owners pay to advertise their apartments and these sites mainly deal with large apartment complexes over 100 units. A lot of available rentals, especially in Los Angeles, Santa Monica, and West Hollywood, are in much smaller buildings and those owners generally don't pay to advertise. They use classified ad websites like Craigslist or hotigloos.com which are free and have thousands of rentals in large cities. Those sites should be checked daily as new rentals appear. Some sites like oodle.com and hotigloos.com offers free email alerts so that visitors can save time and get emails of any new listings they'd want.
Property managers also have websites which show all the vacancies they currently have. For example, the website for Century West Properties in Los Angeles is rentcwp.com. Some property management companies are well known, but others you'll have to search for. In cities like New York and Boston, brokered apartments are very common. Either the owner or renter pays them a commission or fee. They have access to many listings which they publish on their own websites or Craigslist.
People still find apartments in the offline world. "For Rent" signs in front of a building are everywhere, so drive around the area you want to live. That takes more time than browsing online but not every vacancy is posted on a website. Classified ads in print newspapers also have apartment listings, so look there too.
If all these free resources are still not enough, renters can pay for prepaid listing services which charge a fee for a list of available rentals. A popular one in Los Angeles is Westside Rentals. For $60 you get access to their listings for 2 months. Property owners generally list their rentals on free classified ad sites in addition to paid listing services.