You will need a return airline ticket and a visit/tourist visa to enter. Check with the Qatari Embassy on requirements for South African passport holders (sorry, I can't help there).

Be aware that staying with your boyfriend without benefit of marriage is considered adultery and is punishable by arrest, jail, possible lashes and deportation. That said, if you are both white and discreet - it shouldn't be an issue. HOwever, you should know that if there is a nosy and vindictive neighbour - they can report your "bad" behaviour to the police and the police will act on complaints.