I have several rental properties.
Yes, you must declare rental income. However, the cost, such as mortgage interest, depreciation, tax, repair expense, and property management expense (such as gas mileage, office supplies, and phone bills, etc) are subtracted from the rental income. In some competitive market, you may end up with lose, and all you got is the equity on the house, and the cash flow is zero or negative (i have one just like that). Not many rentals have sizable positive cash flow unless you bought at a really low price,- way below market price,
If you bought your property at fair market price, chances are your taxable income (after all deductions) is fairly small, if any.