First of all, welcome to RSCVanilla!
If you are using android, make sure you have the latest java version installed. Once done, download the APK and install it. Make sure your phone allow installation from third party.
If you are using PC make sure you have the latest Java version installed. Then simply open the JAR and the client will extract the game on your computer.
Once you have the game installed, enter your character name/password (Which you create on the forum under character management)
If you find any bugs or have an issue, make sure you let us know through our Bug tracker or by joining our Discord