Get a good idea of the right "types" for each part in your film. Ask yourself what qualities a given role requires. Perhaps one needs a short, stocky man in his 40s, while another requires a thin woman with a nervous tic. Write down the qualities for each part in a series of short bullet points so you can refer to them easily as you go through the casting process.
Determine your casting budget. Some actors might be out of your ideal price range and if your heart is set on a particular performer, you want to include enough money to attract him. You might be able to cut corners casting other parts (using friends or family), but it's important to know the parameters of your budget before you start.
Contact talent agencies and management companies in your area to ask about available actors. Send them your script and ask if they have any actors interested in working in your film. Be prepared to do a lot of legwork on this; finding the right actors can take a lot of searching. On higher-budget productions, a casting director can take this burden off your shoulders.
Advertise in local trade magazines and canvas theatrical companies for interested actors. Also contact theatre departments at nearby colleges and universities; talented students and teachers might not be averse to taking a part for comparatively low pay.
Schedule auditions if you're having trouble casting a particular role. This will allow you choose from a large number of performers.