I've never used Overdrive, but looks like you have to install the app from Playstore on your phone, do you know how to do that?
I found these instructions on the internet, ignore the Suffolk bit (unless you live in Suffolk!) Good luck!
Setting up OverDrive
On a smartphone or tablet with the Libby app
Download and install the ‘Libby, by OverDrive’ app from the App Store, Google Play or Windows Store
When the download is complete, press ‘Open’
When the Libby app opens, press the green/blue button that says ‘Hi!’
A new page will appear. Scroll to the end of the page and select ‘Find My Library’
The app will then try and find your local library service – this takes a few moments. When the app has located a local library service it will ask you at the top of the screen ‘Is this your local library?’. At the bottom of the screen you will have an option to select ‘Yes’ or ‘No’
If it has located Suffolk Libraries, press ‘Yes’. If it has located a different library service, press ‘No’
If you have pressed ‘No’, a new screen will ask you if you want to search for your library service. Select this option
Type in the box ‘Suffolk’
The app will automatically bring up matching libraries. Select the first option shown, ‘Suffolk Libraries’. You now have access to the new Libby OverDrive ap