CLICK HEREthe link above is for a civic but may apply to you also.
first let me explain the problems you could be having,
first check the seatbelt is not jammed in the door get in the passenger side and check the seatbelt is not stuck in the door if it is try pushing the drivers door shut further then pull the handle and give the door a hard pull open this usually works if the seat belt is jammed in the door.
the other problem you could have is mechanical failure of the locking mechanism if this is the case then you need to remove the door panel the best you can as this is the only way of opening the door.
the other problem could be that the locking rod from the locking mechanism has fell off and is keeping the door in the locked position, this usually happens if the door has been slammed shut or the car been hit by another car, the only solution is to remove the door panel.
its going to be hard work but you need to remove the door card if you can do this without damaging it then this will save you some money if not then your going to need to get a new door card/panel.
Now if you take it to a garage they will not mess about taking the panel off they will just cut through it and charge you the labour plus the privce of new door card/panel.
sliding the seat forwards and backwards will help you get to some difficult screws. you may damage some screws if you can get to them but this is the only option you have
have a good look at the passenger side door card/panel to see the work involved, their maybe some hidden screws in the door handle that you pull to close the door so you may need thin screwdriver to remove any caps on it, also the handle you use to open the door from the inside you will need to hold this in open possition to get to the screw that holds this handle on.
have a good look at passenger side to know where the screws are holding the panel on
other than that their isnt much you can do, unless you have the tools to pop the lock from the outside.
try using google and type in how to get in my locked car without the keys this will then list loads of sites that tell you how to make tools to open the locks from the outside using a metal rod down the window, this wont work if the problem is down to mechanical failure.
BUT BEFORE YOU TRY ANYTHING TRY PUSHING ALL YOUR WEIGHT AGAINST THE DOOR FROM THE OUTSIDE AS IF YOUR TRYING TO PUSH IT CLOSED FURTHER THEN WHILST YOU STILL HAVE YOUR WEIGHT ON THE DOOR PULL THE HANDLE TO OPEN THE CAR THEN LET YOUR WEIGHT OFF THE DOOR AND PULL IT TO SEE IF IT OPENS sometimes if its not closed properly it can jam so push hard against the door as if your trying to close it further and keep the pressure on then get the handle and pull it in open possition then let your weight off the door and check if it opens doing this will sometimes get a jammed door open
let me know how you get on or if you need further assistance ok