A good Project Manager will look after the Project Owner's interest first - while ensuring that the project team is enabled with sufficient resources to get the job done on time and on budget and within the quality requirements for the project.
There are different styles and levels of Project Managements including;
- Senior Project Managers
- Scrum Managers (for Agile type projects)
- Planners (for projects that don't change scope every two weeks)
- Planning Technicians (the ones who actually know what's happening.)
- and more;
A very generalized job description for a Project Manager can be found in the Human Resources section of the Project Management Special Interest Group. However - every project is different and you may want to "tailor" the job description there to the needs of your project
addendum:
If you are the Project Owner - then you should find a suitable Project Manager first;
"suitable" as in - your Project Manager (PM) should be the one who knows what kind of resource people it will take to get your project done on time, on budget and with the biggest bang for the buck**. Let the PM deal with selecting people, negotiating contracts, cracking the whip - etc. - all you have to do as PO - is sign the cheque.