Since and for both introduce periods of time.

=> Since refers to the time period when something began. Use since when referring to a specific time period, time of day, or date:
◼ since 1995
▶ since 11 a.m. yesterday
▶ since last year
▶ since the Renaissance

Examples with since:
▶ We've been living in Chicago since 1996. (specific time = since)
▶ I've been in Paris since last Monday, and I'm leaving tomorrow. (specific time = since)
▶ Jen has been watching TV since 5 o'clock. (specific time = since)
Note: You will never use since + ago. We've been running this business since last year. (NOT: since one year ago)

=> For is used to express the duration (or length) of the activity.
▶ for two years
▶ for an hour
▶ for decades

Examples with for:
▶ We have been living in Chicago for ten years. (duration = for)
▶ I'll be in Paris for a week. (duration = for)
▶ I'll be out of the office for several hours. (duration = for)

Common expressions with since and for:
▶ Joan has been studying Chinese for ages, and she still doesn't speak it well! ( for ages = for a very long time)
▶ We'll be staying in this apartment for the time being. ( for the time being = for now; for a while)
▶ Since when do you wear perfume to school? ( since when = When did you start doing that?)
▶ Ever since you told me that Cindy likes to gossip, I haven't told her anything. (ever since = starting when; since the time when)