Generally, for estate tax purposes, you must file Form 706, United States Estate (and Generation-Skipping Transfer) Tax Return. If death occurs in 2009, Form 706 must be filed if the gross estate is more than $3,500,000. The estate tax is currently set to be repealed for decedents dying after 2009 and before 2011.
If you must file Form 706, it has to be done within 9 months after the date of the decedent's death unless you receive an extension of time to file. Use Form 4768, Application for Extension of Time To File a Return and/or Pay U.S. Estate (and Generation-Skipping Transfer) Taxes, to apply for an automatic 6-month extension of time.
If you believe the estate tax may apply to the decedent's estate, use Publication 950, Introduction to Estate and Gift Taxes.