The method to remove a stain from a granite countertop will vary depending on the specifics of the stain. Without knowing more about the stain I'm leery about suggesting any possible fixes. With granite you have to be careful what you try because you may make the matter worse....
Hopefully this reply will point you in the right direction even if it doesn't actually solve the problem for you. Here are two resources that may help you in restoring your kitchen countertop:
1.) Contact the place you got the granite from. They may be able to offer you suggestions on cleaning the countertop and removing the stain. For a fee, they may be able/willing to come out and take care of it for you.
2.) Check out this PDF from the Marble Institute of America (a.k.a. MIA). It's a great resource that a lot of stone fabricators and facilities refer to. It can tell you more about removing the stain based on what kind of stain it is exactly. http://www.marble-institute.com/ston...care_clean.pdf It's also just a good reference to have on hand for general stone maintenance.