Happy for both of you to decided to get married for answer your question, you need to know first where you will get married? If you choose Belgium, there are 2 ways. 1. You take care the marriage visa from your country or 2. You bring all the documents and submit that to City Hall of your boyfriend.
For more information about Visa from your country, you need to check the website of Belgium embassy in your country.
I hope this little information can help you, dont hesitate to ask again if need more detail information.