I was in the same situation last year when we got the Marketo license and didn't know where to begin.
One thing that I was looking for is someone who would be a partners to advise and guide our organization in the right direction, not just someone who will execute on campaigns. There are plenty of people who can do execution part. The trick for me was someone who will build a strategic plan.
Since, I've engaged with HyperX Media out of Salt Lake City. Not only did they help us get of the ground with Marketo, they helped tremendously in Marketo/Salesforec integration and helped us even improve our SalesForce Operation. Furthermore, I recently transferred all of our content and SEO work from tow of our vendors over to HyperX. All because their true partnership approach to our relationship.
I wish all vendors were more like partners.
