I am also a month old in Doha and managed to find an apartment through Nelson Park. They were very good, and actually seemed to go out of their way to help.

Nelson Park is independent and so you will have to pay them. Others, it depends I think. Some agents actually work for the landlord, and you might not always have to pay a commission. Problem with that is, you will only see their properties.

For me, using an agent was good, expensive, but good as I didn't know anyone or anything about Doha. Try Nelson Park.