The IRS recently published guidelines for what is required in order to claim the First Time Homebuyer or Repeat Homebuyer Tax Credits.
For both credits, you will need to complete IRS Form 5405 and submit that with your tax return. In addition, you will need to submit a copy of your HUD-1 Closing statement. The closing statement should ideally be the copy from your closing with both seller and buyer signatures on the page. You can find a copy of form 5405 on the IRS website at http://www.IRS.gov.
If you are claiming the Repeat Homebuyer Tax Credit you would need to provide evidence that you owned and resided in your previous home as your primary residence for at least 3 of the past 5 years. Examples of documentation could include utility bills, property tax statements, mortgage payment stubs, or home insurance bills.
Of course, if you have any questions you should seek counsel from a CPA.