Start with the pro forma the seller gives you, then delve into the numbers.
Utilities will be one thing. Even if tenants pay some, you'll have lighting for common areas, potentially heat, trash removal, maybe snow removal (depending on the area), landscaping, etc.
Find out how much management in your area typically runs - definitely don't want to be "cash flowing" only because you're doing the management yourself, rather than paying someone. Glad you realize that, cause many people don't get that fact.
You'll need to figure out how much needs to be put away monthly for CapEx (future capital expenditures).
Maintenance expenses.
Insurance.
Taxes.
Pest Control.
Bookkeeping.
Advertising.
Cleaning.
That's off the top of my head. I'm sure there's more, and if you google it, you'll probably find some. Hope that helps. :)