If your door is sticking, this does not necessarily mean that you need a new door. It is likely that your door and/or lock needs adjusting and the mechanism may need to be serviced.
If the door is sticking so badly that the door will not lock, you may need a replacement lock but you will usually still not need a new door.
The best thing to do is to call a locksmith before the problem worsens, to prevent being locked out. We can give quotes over the phone and never start any work until you are happy with what we are doing and with the price.