Since you're a small company that has never used project management, it's better to get a project manager with a proven experience in managing projects. Generally, project managers know what to do themselves:
- Manage the requirements (gather the requirements, translate the requirements into the project plan)
- Create the project management documents
- Manage the project (ensure it's on time, on budget, on scope)
- Manage the resources (assigning resources, ensuring resources are completing their tasks, resolving conflicts, handle motivational issues)
- Manage the stakeholders (that's you) and accommodating their requests
- Manage change (including creating the change control)
- Manage project procurement (vendor/contract management)
The above are the standard responsibilities of a project manager. But for a project manager to COMPLETELY know what to do and how to do it, you need to explain to him the following:
- Who does he report to?
- What is your most important and critical constraint? Is it time, money, or quality?
- What is your organization's type?
- Does he have authority over the resources?
- Will he be responsible or accountable for the project, or both?
- Do you usually intervene in the projects?
- Do your project experience heavy changes because of the client?