Your best bet is to go to Qatar Investment Council or Ministry of Economic and Commerce.They're not too far from the city centre i think.Just google them and you will get their full contact info.
I'm responding to your post to warn you of some people on this forum/QLers that pretend they can get your company registered but beleive me they are only after the 20%(you will end up paying for their service) of your initial investment capital.So be WARNED!
Go directly to the parastals(mentioned above) in charge and get the necessary and correct info from them.Never try to cut corners cos there are wolves around waiting to devour your hard earned cash.
A WORD IS ENOUGH FOR THE WISE!!!