Use for with periods of time: I've been studying English for two years.
for...
three years
two weeks
four days
five hours
ten minutes
three decades
two centuries
Use since with measuring the time after a specific point in time: I've been studying English since 2010.
since.
9:00
1973
Monday
February
last Christmas
I was a child
I graduated from college
As you can see from the last few examples, you don't necessarily need to use a specific date with since. Let's say I graduated from college in 2004 and now it's 2014. I could say:
I've been working at this company since 2004.
I've been working at this company since I graduated from college.
Again, since measures backwards to a specific point in time.