OsmAnd is a mobile application available for Android and iOS. You can download it on most popular Application Stores.
iOS: App store.
OsmAnd is an offline map application, so after 'Get started' screen, you will be proposed to download map of your region. If you skip maps download (via 'Skip'), the application won't function correctly. So you will need to download maps later via Android / iOS -> Menu -> Maps download (TODO). OsmAnd might not detect your region to download correctly, so you can choose from the list.
Android storage. On Android devices your might want to change where the maps stored, on shared storage, sdcard, etc. You can read more about it on Storage.
In case you need to reinstall the app, please make sure you have back up all your Personal Data data such as Tracks and Favorites.
TODO: Give location permissions / start first navigation / select voice navigation.