To make things as simple as possible.
Make sure you have a solid internet connection, and that you are not downloading anything or has any other program or device that eats your internet.
Then just keep trying.. Over, and over, and over until it works or possibly wait to a later time, try other characters in other maps etc.
I'm sitting on a fiberoptic line and sometimes there comes days when the plains are completely full of users which requires me try to connect to the server around 10 times before I actually get inside that map.