Most property agents and Landlords will as a minimum require satisfactory references from your employer (or college), and a previous or current landlord. They also may require a personal reference, and sometimes a guarantor. In addition a credit check will be undertaken. Some may use the services of an independent referencing company to obtain and evaluate these references.
It would be advised that agents sometimes charge for this service and the cost at times is non refundable. Please make sure you have adiquate means to rent the desired property and know your back ground credit scoring thus avoiding embaressment and delay.
If in doubt always phone the Agent/Landlord who is listing the property and ask as many questions as possible regarding referencing, as not all agents have the same process.