If you're looking to add any extra functionality to your store, then you will most likely want to consider adding some apps (which act as add-ons to provide extra features for your store).
Before adding any apps, I highly recommend making a duplicate copy of your theme, so that you always have a version to revert back to, without apps added. It's highly unlikely that any apps will break your store by adding them, but it's better to be safe than sorry!
You can browse all of the apps that Shopify has to offer by going to the Official Shopify App Store. Listed below are my recommendations for apps to check out. Only add the apps you actually need (try to stay under 10 apps total for the best performance and loading times).
Note: I've added a star icon next to my most commonly recommended apps.
Pop-ups & Messages
Coupon Pop - Boost your store's conversion rates by offering your customers discounts for joining your mailing list or liking/following your social media pages.
Announcement Bar - Show an announcement (ex. shipping, discount codes, etc.) at the top of your website.
Gamify - Fun and entertaining way to convert your visitors into email subscribers and grow your emailing list.
★ Photo Resize - Make all of your product images the same size for aligned collection pages.
★ Minifier - Optimize your product images to decrease loading times.
Security & Protection
Signifyd - Greatly reduce your chances of losing money to fraudulent orders (offers a "no chargebacks" guarantee).
Age Verifier - If you sell age restricted products such as alcohol, tobacco, cigarettes, spirits and more, use this app to verify the visitor’s age.
TRUST - Decrease your customers’ perceived security and privacy concerns.
★ Locksmith - The best way to lock sections of your site to only be accessible by approved customers. This app also supports monthly subscriptions.
Increase Sales & Conversions
Limespot - Show your customers products they are more likely to buy and increase both the average basket size and conversion rate.
Kit - Drive more sales with your own automated "marketing employee."
Customer Support & Live Chats
★ ReAmaze - Live Chat Customer Service Helpdesk + Chatbots & Help Center.
HelpCenter - Build a solid, attractive, and branded FAQs page.
Tawk.to - A free live chat application designed to increase the effectiveness in managing the online customer engagement experience by chatting with live visitors on your site.
Zendesk - This app is the world's best premium live chat and email ticketing system available. It's what I use for Stephen's World, and what Shopify uses for their support team. Get your first year of Zendesk for free by clicking here.
Improve Customer Experience
★ Geolizr - 4 GEO TARGETING APPS IN 1: currency conversion, notifications, popups, redirects - all based on the visitor's location, and is highly customizable.
Social Login - Easier and faster login with Facebook, Google, Twitter, LinkedIn & Amazon.
Scroll2Top - People can move to the bottom of the page to see what they like, and instantly return to the top when they want with just one click.
Shipping Tracker - Keep you customers updated about their packages locations.
Wishlist Plus - Let customers save products they're interested in using a wishlist.
★ Judge.me - Site & product reviews (photos, videos, Q&A for social proof).
Product Reviews - Increase the sales of reviewed products, and add SEO-friendly review scores to your Google search results.
Product Reviews Addon - An add-on to Shopify's Product Reviews app (must already have the app added). Provides more in-depth features, and helps increase your social proof.
Order Printer Templates - Templates for professionally designed Invoices, Packing Slips, Return Forms and Gift Receipts that match your store’s design.
Customer Pricing - Offer wholesale, or customer-specific pricing to premium customers. Manually tag customers into groups, sell memberships to the special pricing, or set the app to do it automatically based off how much, or how often, customers order.
★ Wholesale Gorilla - Simple and powerful wholesale functionality for your shop.
★ Product Options - Product Options & Customization w/ Conditional Logic & Pricing.
Need more help with this step? Check out Shopify's help docs for adding apps by clicking here.