Using Federal tax form 1040 I entered a capital gain (from sale of stocks) on line 13 and added it into my total income for line 22. My total income is circa 30,500 including this gain. I've heard that capital gains are not taxable for certain income brackets. If capital gains are not taxable, where on the tax form 1040 is this reflected ? It would greatly lower my taxable income but I don't know where/how to show this. What am I missing?

Form 1040 is here --> http://www.irs.gov/pub/irs-pdf/f1040.pdf

You need to compute your tax using the Schedule D worksheet.

If you are using tax software, the software will do it for you.