Need some informations.

hi!

At first, you need to install the appropriate server software (in your "my server" panel. There you need CraftBukkit or Spigot 1.8.* (I'm not sure if the plugin works with 1.9 because there's no information if the 1.8 plugin version is also working on 1.9)
Then you need to go to the "Plugins & Files" page (you find it in the drop-down menu next to you username). There you click on "download plugins", search for "chairs" and the only plugin which pops up is the one you want. Just click on it and choose the version you want (the best would probably be the newest, but if you want to run a 1.7 server you need the 1.7 one, of course).
When you start the server the next time it should automatically install itself and you'll find some files from the plugin in your "plugins & files" panel. You can use the config file to change some settings of the plugin.

If you need more detailed help with the plugin itself, just watch this.