I forgot to deduct my HSA contribution in my 2012 tax return. How can I file an amended return and claim the HSA deduction?
Did the W-2 you received from your employer have a value in Box 12 with a code of W?
If yes then the company already accounted for their contribution and your pre-tax contribution on the W-2. You should have submitted form 8889 and instructions. If you used software to complete your tax forms this should have been done automatically. Your contributions and the companies contributions should be on line 9.
If No, this is more complex because your contributions were made after tax, because your contributions would not appear on the W-2. You will have to gather the documents from the HSA administrator. Those contributions not made via paycheck withholding are entered on line 2 of form 8889.
If you didn't include form 8889 you will need to file an amended return.