I had a client who had an app for their restaurant. I found it quite hard to find how to get a link to the app to put on the website. In the end it was quite easy. From my iPhone (or iPad would work too) go to the app store.
Then find the app and click the 3 little buttons to the right and choose Share App. Then mail to yourself and you will get a link to the App. You can stick this link on your website and then mobile users will be able to get the app from your website. Android Apps are much easier to get the link. Just Google the app. Search in the app store and copy the address from the address bar.