When you Create a New Program, one of the steps required (Audience Criteria - you will be prompted) is for you to upload a list of the contacts you wish to reach out to. You can add contacts in bulk via CSV file (a format for this is provided for you in the application) or one at a time.
If the Purpose of your program is Qualifying or Servicing Inbound Leads that come to you via a landing page form for example, you will need to connect a lead source (so that leads flow into your program sequence for execution as they come in) - follow the prompts in the Audience Criteria step of the Create a Program process.
See also FAQ: What if I don't have a list of contacts that I can reach out to?
See also FAQ: Do I need to find my own list of contacts to reach out to?