If you need an exact copy with all the attachments you will need to fill out Form 4506, Request for Copy of Tax Return. On the form is the address of where you will need to mail the form. Include a check or money order for $57. If you need one from the year before, just include another $57.00. You can go back as far as six years. Make the check out to United States Treasury.
If you don't need an exact copy, you can get a transcript. This will show most line items including forms and schedules. It is usually all you need for lenders or mortgage verification. The form is 4506-T or you can even call 800-829-1040 and following the instructions from the recorded message.
If you just need a copy of the changes you made after the original tax return was filed, you can request a Tax Account Transcript which shows marital status, type of return filed, adjusted gross and taxable income, and the adjustments you made. These are available for the last 3 years.