To get started you need Riot on your Smartphone or Computer. It's possible to be logged in on multiple devices.
The most easy way is to use Riot on your computer, you don't have to install anything, you can just log in on https://riot.asra.gr.
If you have F-Droid you can just install it from there, therwise it can be downloaded from Google Playstore. It's also available for I-Phones on Appstore.
Important: On your Smartphone you have to input https://asra.gr under custom server two times.
If you don't like Riot, you can choose a bunch of other Matrix Clients.
When you got Riot running and don't have an account yet, you can create one.