Before you spend on more traffic, make sure your store converts the traffic it already has. A few focused fixes frequently pay for themselves in weeks.
Product pages
Use sharp images from several angles, clear pricing, honest descriptions and visible reviews. Answer shipping and returns questions on the page itself.
Checkout
Offer guest checkout, show total costs early and support the payment methods your customers prefer. Every extra field costs sales.
Speed
Compress images and trim scripts. A store that loads in two seconds will almost always outsell the same store loading in five.