I agree with Herman, this has to be driven by the question, "What's the Goal?" - The more specific you can be about the exact value-add to the agency, the better. What before How, just as Herman said, but even more than that the goals have to be measurable and objective.
I've seen too many PMO's even at the program/project level that just become another layer of bureaucratic overhead and end up hurting instead of helping the agency/organization. Without a clear definition of value and the related objective metrics, it's really impossible to tell if you've created a blessing or unleashed a parasite on the organization.