Depends on if you in the Military or ther organisation approved by the Be Tax Authorities or are employed on the Be civvy net.
I am not at SHAPE but up in Bxl, but the principal should be the same if not the detail.
If you are the former or the dependant of an entitled person you need to speak to the relevant office in SHAPE who should give you a standard letter (approved by the tax authorities) that you include with your tax return (uncompleted but stamped by the SHAPE office) when you return it. The letter explains your tax status to the authorities as you are not usually liable to taxation by the Be authorities.
If you are locally employed then you had best employ an accountant to advise you. Most people I know do this if they are not fluent in Fr or Dultch.
Again local taxes that are the equivilent of those covered by Council tax should be paid by the relevant office in SHAPE, we recently received a bill for "green" taxes which the office in HQ NATO paid.
Check with the correct office in SHAPE for details, do be prepared for some of the most archane rules and regulations if you thought things were complex in Germany they have nothing on the Belgians.
Whatever you do do not ignore the return, as if you do not jump through the correct hoops they will fine you even if you are not liable to tax as you have not followed the correct proceedure even if you do not know what the procedure is.