'For' and 'Since' with Present Perfect

We use Present Perfect tense to talk about action which started in the past and continues up to the present.

I have had this computer for about a year. 

How long have you been at this school? 

I haven't seen Julia since September.

We use 'for' with a period of time, for example: a few days, half an hour, two years.

I've been at school since 8.30 a.m.

We use 'since' with the time when the action started, for example: last year, June 8, I met you.

I've been at school for 5 hours.

Many links to possible exercises