For and since are prepositions . The preposition 'for' denotes duration of time.
For example: I watch television at least for one hour every day.
Since: It is used to denote the action when an activity had begun.
For example: I have watched television since morning. I have watched television for two hours.
Please note : The above prepositions are generally used with the perfect tenses and should never be used with the present tense or the present continuous tense. If you say - I am living in Mumbai since 2000, it is wrong . You should either say: I have been living in Mumbai since 2000 or I have been living in Mumbai for last ten years.
Raj has been playing tennis since his childhood.
Rani has worked for this company for last three years.
Please switch off the air condition, it has been on for more than five hours.