If a few units, a spreadsheet will suffice.
Column A: all rents collected
Column B: all expenses, summarized by sub-items such as property taxes, mortgage interest, insurance, R&M .. then give this to your tax filing person/accountant once a year, or if many properties once a quarter, as some items may not be expenses actually but capital expenses (say a new front door or a new furnace) and the mortgage interest is different from the actual monthly payment that is comprised of principal and interest. You can do that yourself, of course, but it is a distraction from the goal.
Excel has some nifty templates that you can easily adapt for your purposes.
Don't major in minors, though.
Your main job as a real estate investor is threefold, with a focus on the first two:
a) find great properties to buy in area with rental and value upside
b) find more JV partners to invest with you to buy more properties
c) manage them impeccably, but this task can be outsourced for a fee