Flipping a house with cash or credit

I’m just starting out with wholesaling and I’m a little confused on some of the processes. So I’m confused as to how I put a property under contract without any upfront money. All the information I read says that you just need to put the house under contract but everyone I’ve spoken with says there needs to be money put down. Am I missing something or do I just not understand the process correctly? I’m in Baltimore, MD, not sure if that makes any difference.