How it works:
You first need a sense of how much you can afford to spend. Will it be all cash, or involve borrowing. Either way, you need a budget. To buy four or fewer units, you'll need 20% down to get the best loans, and at 5 units or higher, 35%.
You have to then find areas you can afford with prices that will cashflow well. If you have a hundred thou, I would look for a building that costs $400,000 or less and grosses over $3400/month. Figure out the loan costs and tax, insurance, and upkeep costs. Figure out if the expected rents cover that, and if there is a profit. If you like the numbers, go for it.
Tips:
Buy in areas where people with young children live; these are always crowded and popular
Rent to good people with difficult pets; the damage is minimal or non-existent, but they pay more and are more loyal.
Try to gross 10% of your purchase price per year, or better.
Minimize landscaping effort with low maintenance yards
Always go with your gut and watch for red flags
Do not discriminate against types of people as renters. Use credit scores to do that.
Be prepared to tell tenants why you are rejecting them; in some states they have a right to know. Do NOT discriminate against children above all else.
Do not take a realtor's word for it regarding rent. Run a bogus ad trying to flog some of the proposed units and various rents and see if anyone calls. DO NOT LISTEN TO REALTORS.