Use for with periods of time: I’ve been studying English for two years.
Use since with measuring the time after a specific point in time: I’ve been studying English since 2010.
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:
Again, since measures backwards to a specific point in time.