Is there a way to figure out or a website to see how much fuel you will need for your flight?

Read the tutorials. They explain how to do this with the CDU.

you can use my fuel planner : http://library.avsim.net/esearch.php?DLID=175850

 

or another more complex : http://forum.avsim.net/topic/405110-free-alternative-to-topcat/