Since and for are common prepositions used in many statements. However, they are often used in wrong places.

Since is used when one talks about one particular point of time.
It has been raining since yesterday.
Yesterday here refers to a particular day, hence since is used.

For is used when one refers to a period of time.
It has been raining for two hours now.
Two hours refer to a period of time, hence for is used.

Here are a few more examples for more clarity:
Kids have been playing since four o’clock.
Kids have been playing for last one hour.
You have been studying since morning.
You have not eaten for past eight hours.