Once you have downloaded the .jar file it will be in your downloads folder. Just double click it to launch it.
When you get to the login menu, no need to register. Just type in your desired Username and Password to register.
And yes! Everything can be played for free, however there is no questing on most private servers.
If you still need help just say 🙂 It could be easier and faster to get help via our Discord: https://discord.gg/NzShtY (This link only lasts 24hours from this message)