looking for fill up tax form.

Go to the local tax office - details and opening hours on the paperwork which is sent to you.
Take all the relevant papers, join the queue and they do everything for you.
It's free.
You may need to take someone who can interpret for you but in Flanders they usually speak English.

I strongly suggest you take Kasseistamper's advice.
If you go to a tax or accounting firm they charge you a lot of money on an hourly rate. Starting charges can be almost €500+ per hour (not including taxes).