Ideally get a co-founder who is interested in marketing your product. However, if you are single founder like me then technically you could outsource marketing if you have already achieved a product-market fit. These are the steps I would follow before I outsource marketing. (these are very generic steps since there is no information on your product)
1. Find out who my target customer and whether my product solve his problem. This would be done by reaching out to those customers/users who regularly use my product. Depending on customer/product/market this could involve speaking to them on the phone, sending a simple personalized email or an online survey.
2. If I know where my target customers hangs out, I would try to figure out if there is a scalable way to reach out to them. Hopefully there will be multiple channels I could use. I would ideally test these channels to see what kind of conversions I get and at what price. (still just gathering data and not scaling up)
3. At this time, I would look at "retention rate" as the preferred metric to obsess over. "No of visitors per day" tends to be a vanity metric. I would also look at testing out different copy on my landing page to see whether the value proposition resonates well with the target audience (and converts well). Generally the copy wording comes from step 1 - but gets validated with step 3.
Hopefully at the end of step 3 I have a better product-market fit. Targeted users are entering my funnel and with enough retention (and hopefully revenue). This would be a good point to outsource marketing, since you can measure how effective the marketer is.
If all of the above sounds like something you absolutely loathe to do (and that's natural for a lot of devs), then I think the only option is to get a marketing co-founder involved right from the start.