Search Engine Optimisation (SEO) is a multi-faceted topic, all about improving your rankings in Google's search engine listings (and to a lesser extend, other search engines also).
SEO is not necessarily all about increasing sales, though obviously with more incoming traffic resulting from your SEO efforts, that should ultimately be something that happens. Conversion Rate Optimisation (CRO) is all about turning your traffic into paying customers. You can improve your SEO to gain new traffic, but if that traffic does convert to paying customers, the SEO didn't increase your sales, if you see what I mean?
Therefore you should be looking to focus on both SEO and CRO. Increase your traffic, and do everything in your power to make the most of that traffic. Convert as much of it as possible to paying customers. Many stores don't practice CRO, and waste a lot of their existing traffic. They often throw more money at advertising, just to increase their traffic which also fails to convert. In many cases, it would be best to focus on your CRO and improve your conversion rate, before pursuing more traffic.
In any case, SEO and CRO are multi-faceted. There's a lot of different things you can do (both on-site, and off-site) to improve how the search engines perceive your website, and ultimately it's their perception of your website that impacts your search engine listings. The topic of SEO and CRO is quite vast, and there are real experts in the field, as well as others who'll take your money for little gain to yourself. You should be careful when choosing an agency or freelancer, when employing 3rd party SEO/CRO services. As an example, I've seen many times, agencies which charge a monthly fee to carry out SEO tasks on your website. When you decide you don't want to pay them anymore, it's in their terms/contract that they can "undo" a lot of that work and remove their articles. Be careful to avoid that.
That said, you don't need to hire a 3rd party right away. You can certainly make a good start by yourself and do it in a DIY fashion. Here's what I advise for this:
Again, content is king so get that right and you're off to a great start. No agency knows your products better than you do, so write good content, and perhaps employ a 3rd party afterwards to help you turn that content into copy which sells better.
I hope this gives you a good starting point, and remember, SEO is always changing so you're on a never-ending path to improve your listings.