you have to go to town hall to apply for marriage. then they will ask you to fill in all the forms and provide all the documents. then they will send the application to IND for advice. Then IND will come to your house to check if you really live with your bf or not. Then after half year or so, you will receive a notice from town hall if they grant your application for marriage or not. Then you can make an appointment to chat with them about the details of your marriage. Then you will have to pay for the marriage in the town hall. It varies a lot between different town halls...
In randstad, it can cost upto several hundreds euro for half an hour marriage ceremony.