The taxable amount, if any, is the amount you receive that is more than the premiums you paid over the years. If you don't know your premiums, the life insurance company will know, call and ask them.
Depending on your other income, you will likely owe 12% or 22% of the taxable amount as income tax. You could have it withheld or you could make an estimated payment at www.irs.gov/payments.