• Anything under 6 units is considered residential and requires a 20% down payment. You might get 15% down on a six-plex but if this is your first deal, I doubt you'll get that rate.
• You'll need a cash account to deal with closing costs, insurance, transfer taxes, a lawyer, environmental fees (maybe?)
• Don't forget, this isn't passive like a stock or bond. This is a second job. You'll have to manage it or pay someone to manage it for you . Companies charge 5-10% to manage a property. Don't go cheap. Find a good one and pay up to 10%. You probably don't want a large place at first. Get a duplex or triplex and get comfortable. Put systems and processes in place. Let the management handle tenant landlord law.
• Location! Do your research first. Get a clean place that isn't high-end near good schools or transportation.
• Your goal is to produce positive cash flow after covering management/taxes/mortgage/insurance/utilities/maintenance/snow removal/lawn care/liabilities/accounting/bookkeeping. Don't forget to build in a vacancy allowance, repair allowance (~4% of income?). Should we also build in a lawyer allowance? Do not forget to account for present and future interest rates. Use the IRS Schedule E as a reference to map income and expenses to the categories in the schedule.
• Remember, your IRA can legally own real estate if you want it to be part of your retirement strategy.
• Read this when trying to value the property before you buy http://pages.stern.nyu .edu/~adamodar/pdfiles/valn2ed/ch26.pdf
• Assess where the rental/landlord market is right now. Does it favor renters? Does it favor landlords? What metrics decide this? What is the probability of the market switching? Spend time on CL to get to know the local market. Befriend a few agents (meetups?) Pay attention to how long units are listed on CL. Look at rent differentials by neighborhood.
• Make sure anything you buy doesn't have weird zoning or neighborhood association issues (like rules that say you can't rent)
• Is airbnb a good idea? That is a totally different business. If you want to do that, learn it.

Why would you buy a duplex? Not for the cash flow. You're leveraging the bank's money to get 100% of the appreciation by only putting down 15-20%. Of course, if that is your goal, you need to model long term real estate appreciation projections. Remember, you'll be tying up a lot of capital in a long, illiquid play.