In this industry of ecomm, no matter what you sell, you can vary rarely rely on sales to just happen when you open a store. I would start by making sure you have connected to all the feeds like Google Shopping Feed, Facebook Shopping Feed, depending on what you sell, even Pinterest Shopping feed and then make sure that you are leveraging your free marketing via social media, email campaigns and learning about (and implementing) on-page and off-page SEO - you can find hours of video content to teach this. All of these are FREE avenues. Once you have exhausted all of these avenues, you should start to see some sales and you'll want to capture email addresses and continue to stay in touch to keep those customers active on your site. There are numerous paid avenues but in my humble opinion, there is no one app that will just flip a switch to sales for you, that is why many ecomm site owners give it up. It is hard work and takes a ton of energy get your name out there.
After the above, I would look at Facebook or Google marketing but I would count of a professional to help you with this as it can get expensive very quickly with little to no ROI if not done properly.
Hey there! I am the owner of the Arena Shopify app. This app is really great to help you get more sales through social media and email marketing.
With it you can setup a landing page on your website that you can share on social media where visitors can sign up with their email to enter into a giveaway. You can then send them offers on your products to the email they signed up with.
They earn more points when they share the giveaway on social media and their friends enter into the giveaways as well, creating a viral feedback loop of more traffic to your store. You can even give them a discount which can be used immediately when they sign up to get sales through your giveaway.
It's a great tool to use when you have a low marketing budget to get your first few customers. I really recommend giving it a try