I echo Andy’s advice and would add this from the marketing agency side.
1 – Pick an agency who has already worked for a similar client in your industry. They will have experience and you won’t be teaching them the basics
2 – Manage them very closely including requiring they share logins for paid advertising (Facebook & Google) so that you can independently check their work
3 – The detail in the analytics is huge. You must have expertise yourself so you can see if what is being marketed is what you are selling; so there’s no place for mis-judgements between strategy and tactics. I can’t emphasise this enough.

If you don’t have marketing expertise in-house, you are heading down a risky road where you could waste a lot of time and effort for little reward.