For and since can be confusing for people learning English.  They both are used for talking about time/time periods.  Let's take a look at how to use them correctly and some examples of usage.

First of all, "for" is used to show a period of time.  Examples of periods of time would be 27 minutes, 3 hours, 6 months, 2 years, etc.  To form these sentences, you can many verb tenses.  So after reading that, let's look at some examples:

"I studied there for 6 months."
"Claire has been living in San Francisco for over 10 years."
"They work in their garden for 20 minutes every day."
Now let's take a look at the word "since."  This word is used to emphasize the start of the period of time something has been happening (or in some cases not happening!) for.  Examples of this would be since last year, since yesterday, since last Wednesday, etc.  This tense usually requires the use of a perfect tense.  Here are some examples in actual sentences:

"I had been living there since last July and then I moved here."
"Jimmy has been learning how to drive since last week."
"Many people have visited our shop since we opened last month."