These two words are commonly misused, even by intelligent, articulate writers and speakers. Here's how to use them correctly:
Because is used when providing a reason for something. "I went to the store because we needed milk."
Since is used when dealing with time. "We've needed milk since this morning."
Unfortunately a great many people use since as a replacement for because. I'm not sure why, but that's the way it is, which leads me to think that eventually, if it hasn't happened yet, the two words will be considered officially synonymous. Bummer, man.