Hi there "Freek" (may I call you that)
Seriously though... How about a few more details? Like dimensions to start with!! Any isolation issues, what's on the other side of the walls/ceiling? What's the construction of the walls and ceiling? What type of "market" will you be aiming at, what type of sessions do you envisage? etc etc.
It won't be long on this site before people will start asking you for measurements. So, get yourself to REW - Room EQ Wizard Home Page
and download a "free" (...yes free) copy of Room EQ wizard. Learn how to use it. Make your first measurements in your nice clean empty room as a reference.
If you only buy and read one
book, make it Home Recording Studio- Build it like the pros, by Rod Gervais. It will sort out a lot of mistakes, before you make them, and raise a lot of questions that you probably hadn't thought to ask.