My car door won`t open!!?

Hi I wonder if anyone can give me some advice. My front passenger side door is stuck closed. I`ve tried to take the door card off to get to the locking mechanism to no avail.
I really don`t want to have to take it to a garage cos I`m pretty sure it`ll cost a fortune to fix.
It`s an 07 Fiesta.
Any suggestions about how to open the door or fix the fault will be gratefully received.
Please help!!