What differentiates your store from any other store that sells the same items? Price? Brand? Do you manufacture the products yourself?
Once you know these questions you need to focus on your customer. Who is your customer? Build a persona. If it's too broad or general you may have several different target customers, like Men aged between 20 and 30 or women aged between 30 and 40. Basically get to know your customer. Who are they? What are the demographics?
When you know your customer and what they want you can then target those customers through advertising. And you have to test and work on your click throughs against your target customers and persona. when the CTR is a strong one for example for every $10 you spend on advertising you make $50 profit through sales then this is generally a good start.
At the end if the day the key reason why a customer will buy from your site or any site at all are the questions on the first sentence. If your product is different tell them why in the ad, if its cheaper than a competitor tell them why, if you stock brands, make sure you have permission from that brand to advertise them on your site. If you manufacture these products yourself this is one answer to what differentiates you from competitors. but if you sell the same as what Nike or Adidas sells then its about building your brand - make it cool.
