Start by looking for lower-end houses that are in good condition. Then, start looking for a bank that allows you to put no money down. These loans have become a lot harder to find since the Recession, but some banks are starting to make them available again. In many cases, you may be able to use an unconventional form of collateral, such as your primary residence.
Of course, there are a lot of risks with a business model like this. Using your assets as collateral means that they are in danger of being foreclosed on by the bank. There's also the typical risks that come with renting. Being a landlord is hard work.