Hello
I've been in the US since Fall 07. I am on a F-1 visa. I filed for my tax returns but instead of filing 1040 NR-EZ I filed 1040 EZ. My form has been accepted by the IRS.
What shoud I do now? How do I rectify my mistake?

You will file Form 1040X (amended tax return) with properly done 1040NR-EZ and Form 8843.
On F1 you are nonresident for 5 years and don't pay Social security and Medicare taxes.

Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents (http://taxipay.blogspot.com/2008/03/us-tax-filing-requirements-for-non.html)