Let's start with downloading the plugin. You can do this by clicking here. After clicking the link, you will se a "Download Now" button on that page. Click that button to start the download. This will download the latest version of SkywarsReloaded.
Now we're going to install the plugin on your server. Do this by going to your server files, then click the 'plugins' folder. Now upload the .jar file. Now restart your server or use a plugin like PlugMan to load the plugin (/plugman load Skywars).
Now that the plugin is installed and enabled, you will have to do one lst thing before creating arenas: setting the lobby spawnpoint. It is not possible to create or edit arenas without the lobby spawnpoint being set. So do this using /sw setspawn. Players will spawn here after they left their game or when it ended. They will also be teleported to this spawn when you have one of the options 'teleportToSpawnOnJoin' or 'teleportToSpawnOnWorldEnter' enabled in the config.yml.