So, you want to explode sales, enter new markets, step out in the global arena? You may have gotten this far with zero marketing - in which case, congratulations! It's proof that your product truly rocks and is ready for prime time - ready for a much wider audience.
If you have great ambitions, you will need marketing. So, you talk to marketers but you yourself have never done it and are unsure about what freelancer/agency would be a good fit.