Rae from Shopify here. Thanks for posting this question, and welcome to the Shopify Community!
When you create an online store on Shopify, mobile commerce features are included on your website already. This means that customers can browse and buy from your store directly on their mobile device by visiting your domain. Your store will also look as great on mobile as it does on a desktop computer. You can read more information on this here.
However, if you're looking to build an app for your store, there are some great apps in the Shopify App Store that can help with this, such as:
These are all third-party apps that are developed and supported by teams who are external to Shopify, though. So, if you need any assistance with these apps, or if you have questions about the charges associated with the apps, please contact the app developers directly. You can do this by following these steps.
If you're instead hoping to have someone build an app for your store on your behalf, then I recommend reaching out to Shopify Experts for more information on this. Experts are third-party developers and designers who are very knowledgable and skilled in creating custom integrations for Shopify stores, so they're an awesome resource to consider using. Here's a list of some of the Experts who specialize in creating custom apps.
I'd love to hear more about your plans for this app, and how you're hoping customers will use it. Are you considering offering unique discounts, or other benefits, to app users?
Let me know your thoughts on the above information!