I think you are confusing a few concepts. I live in the US and my perspective are based on American tax laws.
** disclaimer ** I am not a lawyer or accountant ** disclaimer **
You can take the bottled water in your refrigerator and start selling them on street for profit, done, you're in business. But I doubt you'll sell enough water to be an issue to your local tax collector.
Start small and start learning. The LLC, S-Corp, C-Corp etc are legal tax structures. This is needed as you grow but not in the startup phase.
What's your goal, a website collecting ad revenue or start the next Microsoft? Going with a site then I don't think you'll need more than an LLC if that. Going the big company route, then you'll need lawyers attorneys etc, and LOTs of seed capital.
These sites might be helpful:
Starting a Business in Belgium Details - Doing Business - The World Bank Group Essential advice on setting up a business in Belgium, Employment, Belgium, Expatica