If your car is less then three years from purchase, it just needs to be registered with the police after insurance.
Costs, 100 QR.

If 3rd year, it needs to go through a technical inspection:
http://www.qtic-qatar.com/procedure.html

After that, get the insurance done and register with the police as normal, they will give you a slip after inspection. QR 100. Thats it.

Regards,
The slave of Allah - Faisal