Firstly you need to add the GPG Keys by heading to the terminal and executing:
$ gpg --keyserver hkp://ipv4.pool.sks-keyservers.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 7D2BAF1CF37B13E2069D6956105BD0E739499BDB
Then install the latest stable version of ruby
$ \curl -sSL https://get.rvm.io | bash -s stable --ruby
If you want to install a specific version of ruby e.g. 2.5.2, you can run
$ rvm install 2.5.2