I don't have previous customers/testimonials, willing to give a discount if necessary. How do I know which companies need content creation (blog posts, copywriting, whitepapers, email marketing) and don't already have an in house writer?
By talking to them. Welcome to the world of sales! I would like to ask why you started a business with no clue how to find customers, though, as it seems like you've skipped some steps. You need to start reaching out to businesses if you're going to find clients. If you're just bootstrapping, I would start calling and emailing just about anyone under the sun, and trying to network with others in your market. You need to gain a little traction first before you can better consider how to scale.
Identifying your target customer is step one, though, and really should have been done before you even started the business, to ensure there is demand for what you're providing. You also need to have a defined value proposition, especially as you are entering a highly saturated marketplace filled with experts commanding very high salaries, and LOTS of people scrambling for any business they can get.
I'm also curious as to why you started an agency versus freelancing...