You can find your nontaxable distributions on Form 1099-DIV, box 3. These are also called nondividend distributions. They’re uncommon. Since they represent a return of your cost, they aren't taxable.
Subtract the amount of your nontaxable distribution from your basis. However, you might get back your complete cost. If so, report future distributions as capital gains.
To learn more, see Publication 550: Investment Income and Expenses at www.irs.gov.