100% start with Google Shopping / Google ads. That will get you immediate traffic by people shopping for your product, and immediate sales.
Make sure you have email marketing software to start capturing visitors and customers. I like Klaviyo. And I like using an email capture for new visitors, for example get 10% off your first order by entering your email. That will get you increased conversion rate + allow you to start building an email list, which you can use for lots of things, for example if you launch a new, complementary product.
Long term, social media is great for brand building (usually Instagram is a great start but depends on your product / audience and what you want to focus on), but conversion rates are usually lower than for Google ads.
And of course SEO, a good SEO audit by a professional will help you get the foundations dialed in, and help you work on a link building strategy (getting other sites to link to you). I also like blogging to generate content for search engines, write articles / guides for people searching for your product, you can develop trust plus capture them in the pre-buying stage.
FYI if you’re interested in automatically fulfilling Shopify orders with your Amazon inventory, this guide shows you how: How to Automate Shopify Order Fulfillment with Amazon MCF / FBA.
For finding quality people for Google Ads, social media, SEO, this can be challenging. Best place is a recommendation from someone you know. If you don’t have that, you can try Upwork, so you can see reviews about projects similar to yours, other than that, Shopify Experts can give some leads.
Whoever you hire, I would ask for case studies and any articles they’ve written. I’ve found that people that have written on the topic / are thought leaders usually have great experience and do quality work.