You don't really need a special cleaner but if you feel better using one.They sell them here in Home Depot or you can order it on line. The trick is not to leave anything (including water) lay on your counter top for a extended period of time. The sealer can do only so much, after all the granite is natural stone and can be porous. Which can hold stains. For me its worth it for the look it gives.
I hope Home Depot sells the sealant as well. I get the feeling when I'm not home, there'll be 'many' accidents.
But overall I think I'll enjoy having it. Maybe add the bathroom as well. ;-)
BTW re: Nenella's post, is granite treated the same way as marble?
Just make sure home depot sells you the correct sealer! I had a coworker who bought a sealer that a home depot employee recommended to her and she put it on all her new granite counter tops.....it was the wrong product and she had to remove the product which from what I understand never completely came off and was a very time consuming project to try and remove it. You may be better getting a recommendation on a sealer from a local company in your area that specializes in the sealer!
Our granite was pretreated before installation, and about annually I retreat with a product called "Granite Gold" that I bought at Bed, bath, & Beyond (also at granitegold.com). After cleaning and drying the surface, you spray it on, wipe it around for complete coverage, and let it dry, which only takes a few minutes. Works like a charm.
I have used dishwashing detergent, Windex, and a number of other cleaners with no issues.