The simplest way to file your income tax return is with the 1040EZ form, but it's not always the best choice. If you want to write off mortgage interest on your income taxes, it's definitely the wrong choice. You can only claim mortgage interest by filing the full 1040 form, not the short version.
You can only claim mortgage interest as a write-off by itemizing deductions instead of taking the standard deduction. You can't do that on the short 1040EZ form or the intermediate 1040A -- it has to be the regular 1040. You report the interest you claim for the year on Schedule A along with other deductions such as medical expenses and state income taxes. Then subtract the total from the income you report on your 1040 form.
Just having mortgage interest doesn't require you to take a write-off for it. If you decide to take the standard deduction instead of itemizing, you may be able to use the 1040EZ form after all. There are other requirements you have to meet to do this -- for starters, it's only available if your status is single or married, filing jointly. You can't claim any dependents, you and your spouse have to be under 65, and there are income limits, which get adjusted for inflation year to year.
If you don't want to itemize but don't meet the EZ standards, 1040A is less onerous than the full 1040 form. You can use the A form regardless of your filing status, and it offers you more potential tax savings than the EZ. Filing 1040A, you can claim dependents, tax credits -- the earned-income credit is the only possible credit with the EZ -- and several adjustments to income. If you're going with the standard deduction, the 1040A usually works out well.
In some cases, you don't have a choice about itemizing. If you and your spouse file separate returns and he itemizes deductions, you have to itemize too, whether or not it beats the standard deduction. If itemizing is optional, sit down and add up your potential deductions. If it looks like they're less than the standard deduction -- $11,900 as of 2012 for a couple filing jointly -- even with mortgage interest included, you're better off not itemizing.