First step is to find a PCB designer that is suitable for the scope of work.
If you have prior experience with electronics and know exactly what you want, you should discuss the details of the project with the designer to make sure he can do the work.
If you do not have experience with electronics, you will need to find out if the PCB designer will meet your needs. Some designers are strictly focused on board design — that person may not be right for you unless you have another engineer who will oversee the electronics (and software/firmware) design.
Next, find out what tools they use and file formats they support to make sure they can work with any design data you give them (such as 3D models or existing schematics). Submit sample data if there is any uncertainty.
If your design will require attention to mechanical details (housing, form factor, heat sinking, alignment with other boards), be sure to bring this up as well and find out how he verifies those details.
Clarify what the designer will release to you — some designers will only provide the job outputs, but will retain the original design files or will require an additional fee to release it to you.
And, of course, discuss the project costs and the payment arrangements, work ownership, confidentiality agreements, and any of the ther business details that matter to you.