Include an image. It can be an icon of a phone (and maybe a tablet too), or a screenshot/s of your app. You could also include the App Store button, which we can provide you with.
List some benefits – why should customers download your app? ie. Keep up to date, get the latest offers first, shop the look.
Put it somewhere permanent on your homepage, whether that’s on the top navigational bar or the bottom information banner.
If possible, link to a landing page with shots of your app and a longer list of benefits.