If you already have an online store, and are looking for ways to improve it, here's a checklist of 8 quick, actionable items you can do today to grow your business.
1. Set up a HootSuite account to manage your site's social media accounts ~ 20 minutes
HootSuite is your centralized social media management headquarters. It allows you to schedule Facebook, Twitter, and other social media posts hours or days in advance, so you can get all of your social media posting done at once, rather than having to post multiple times throughout the day. 
You could potentially set a single time each week to schedule the next week's social media posts, although you may want to post more frequently than that in order to be able to post about the latest news in your niche. Alternatively, you could schedule a week's worth of evergreen content ahead of time, and then simply post additional items throughout the week about the latest news.
2. Make a list of 30-50 blogs in/related to your niche ~ 1 hour
Once you have this list compiled, make it a point to follow these blogs and regularly leave meaningful comments. After a few days/weeks of leaving comments, the authors of these sites will recognize your name and appreciate your contributions to their conversations. Pick a few of the top bloggers, and really put your effort into building a relationship with them. Reach out to them via social media, email, and blog comments, and proactively look for ways to help them in some way.
This type of networking is powerful, as it paves the way for these bloggers to talk about your site/products and start sending you traffic. They might also be able to give you valuable feedback on your products and how well you are serving the audience in your niche.
In addition, it's much easier to get a guest-posting opportunity with a blogger with whom you already have a relationship. Making friends within your niche can only help your business. In both online and offline business, it's all about people.
3. Make sure you have a clear Call To Action (CTA) on your home page ~ 45 minutes
A CTA is an important factor in leading your potential customers towards a purchase. A large, bright button like "Shop now" or "View details" on the home page will strongly encourage users to browse your products, rather than just clicking away from your site when they arrive. Many sites already have a home page slider like the one pictured below, so an easy way to incorporate a CTA without any custom coding is to design a homepage slider image that includes a button (even if the entire slider image is already a link). A visual button as part of the image encourages them to click! 
This is an example of a call to action that I added to one of my client's websites. The entire image was a link, but the button encouraged users to click through and view the products.
4. Try putting fewer products / items on your home page ~ 30 minutes
You should definitely test this to see if it makes a difference for your store, but I've found that showing fewer items on the home page translates into better sales and lower bounce rates. Having too many options confuses people and makes them unsure of what they should click on, so they leave your site! Try focusing on just one or a couple of products, and as mentioned above in #3, include a nice big Call To Action!
5. Edit your copy to focus on benefits, rather than just features ~ A few hours
Rather than talking about the 29 different tech specs and features of your product, focus on how your product will solve a problem for your customer. When people buy quarter-inch drill bits, they don't want the drill bit--they want a quarter-inch hole! (paraphrased from Theodore Levitt).
Of course, you should still include the important details about your features, but they shouldn't be the primary focus. People buy things to solve problems or make improvements to the quality of their lives, so focus on how your product does that.
6. Add a customer reviews app / plugin to your site ~ 10 minutes
Reviews go a long way in building trust, credibility, and lowering people's reservations about making a purchase. If you offer top-notch service and products, let your customers brag about you to others! If you use Shopify for your online store, I recommend the free Yotpo reviews app. 
7. Make sure you have high-resolution, quality photographs of your products ~ A few hours
Good product photos make a huge difference. If you are an amateur taking the photos yourself, try shooting them in natural, diffused sunlight (in the shade, not directly in the light), without a flash, and with simple backgrounds. Follow the tips here to take better product photos.
8. Consolidate your product categories ~ 1 hour
A good rule of thumb is to have no more than 5-6 top-level categories. The more choices you have at the top-level, the more difficult it is for your users to quickly scan through and click. Don't be afraid to have more sub-categories, though.
This is related to the principle in #4 above: If people have too many choices, they are unsure of what to click on, and many will simply leave your site. Make sure it's easy to quickly scan your product categories by limiting them to 5-6 broad top-level categories, and then allowing users to drill down deeper into sub-categories.
Take action today
You have the ability to make some fast, easy improvements to the success of your online store. Which task will you start with today?