The easiest way to switch WordPress to SSL is to us the Really Simple SSL plugin. You can add this as you would any other plugin.
Once installed a banner will appear telling you then SSL is ready and a button that says 'Go-Ahead, Activate SSL'. Click this and job done.
If you see any errors relating to SSL not being installed, don't click this. Double check your SSL.
If you've just installed your SSL, it may take a little while for the plugin to pick this up so just keep checking back.