To expand a bit upon that answer: First, think about your product mix. You cannot sell everything (because you're not Amazon or Wal-Mart). Figure out which products are most desirable to the market and most profitable for you. Focus on those.
You can launch an online store with a single product and it's better to get running with one product than to wait until you have a big selection. This is true of even simple products like, say homemade soaps. It's better to launch with one scent and start marketing it than to wait until you have all three scents that you're thinking about ready to go. Adding your second and third products just gives you another chance to market -- to blog, tweet, email, etc about your new products.
Think carefully about pricing, too. How does the total cost of your product (with shipping) compare to others in the market? Can you offer a buy-one-get-one and make money? Can you offer free shipping if someone spends a certain amount? Can you give a coupon if someone signs up for emails? These techniques will help turn site visitors into buyers. This is called "conversion."
Think about your website branding and design. You don't need to spend a fortune but you probably want some professional input. While tools like Shopify are great, it's just as easy to build an ugly store as a pretty one. Ugly photos, amateur logos, and poor writing are a turn-off to potential buyers.
Think about support, including refunds. What will your policies be? Make them clear on the website before people buy so they're not angry with you later. Consider being generous with support and refunds, especially when you're new. Reputation costs money to build and being good to your buyers, even unhappy buyers, is a cheap way to build a good reputation.
Finally, think about how you will promote your store. Blogging is imperative and Shopify includes a blogging tool. Use it. If you're into social media, promote your products and blog posts in individual social media posts. Will you buy advertising? Where? Google AdWords is a black hole of spending, so consider that your last resort. Are there other websites or physical locations you could use for advertising? Think about the publications and events your buyers use and consider advertising there. Definitely keep an email list in Mailchimp, allow signups from your website, and email folks when there's something new of interest on your website like a new post, new product, or new promotion.
Don't be afraid to drop products or add new ones as you gauge customer demand. The only way to know what your customers really want is to put something out there for sale and see what happens!