On my 1099R I show no taxable amount in 2a. This is my pension in retirement. I have Federal tax withheld. How do I find out the taxable amount for 2a?

Is the box on your 1099-R for "Taxable amount not determined box is checked" checked? Then this is OK if a bit odd.

In the rest of the 1099-R interview, there will be a series of questions, the point of which is to determine how much of your pension income is taxable.

If you had made after-tax contributions to your pension plan while you were employed, then when these dollars are returned to you, you do not pay tax on them. However, these dollars - called your basis - are spread out over many years, and the point of the questions is to figure out how much is your basis, so how much your taxable income should be reduced.