The Tracker Capture app can be found on Google Play under the name "DHIS2 Tracker Capture".
You can download the APK file directly from the GitHub release page.
Click on the app to open it after the installation is done. This will bring up the login screen.
In the URL field, enter the full URL to the DHIS2 instance (server) for which you want to capture and manage data for. Be careful to enter the http:// or https:// part of the URL as well. In the username and password field, enter the username and password for your account at the given DHIS2 instance. Note that you can use this login on the online DHIS2 server as well, i.e. you will be using the same account as you will use when logging into DHIS2 using a Web browser. After logging in the first time, the app will download meta-data from the online instance.