Work Permit for Brussels, Belgium

Hi,
If anyone can help me with the topic, I would be greatful.

I am currently living in Dubai and planning to move to Brussels, Belgium this year. I have a TEFL Certificate (Teach English as a Foreign Language) and plan to teach English.

To gain a legal work permit when in Belgium, would I require a lettre d'emploi? for example:

If I come to Brussels on a Schengen (Tourist) visa, can I simply go the Commune and ask for a work visa? (Usually, I wouldn't think it is possible, but my Belgian friend told me it was possible and very simple.)

Thanks for any clarifications.