While each people locator site or program works a little differently, you usually enter the person's full name if you have it, Social Security number if available, and other relevant information such as previous address and occupation, before submitting your information request. The program or site then cross-references that information with its records and with what it finds online through social media and other accounts to give you possible matches and whereabouts.
Your results are likely to be more accurate if the person you are looking for has a unique name, or if you have personally identifiable information such as a Social Security number or former address. If you are searching for someone named John Smith, for example, it is harder for any program or service to narrow down the list of potential matches to those who might be the person you are trying to find.
The person search function on social media sites sometimes yields accurate results, because it automatically filters results through mutual acquaintances and considers your location, so the top results are people you are most likely to know or be connected to in some way. A person's profile often gives a current city of residence.
Many people-finding websites, even those that advertise free service, often require a fee or subscription before they provide any information that can be useful. Read all of the terms and conditions for the site before you provide any financial information or request a subscription.