One of the great things about Minecraft is that installing basic plugins are simple. What are basic plugins? Basic plugins are ones where you do not have to configure; just install and its ready to use. There are some more advanced plugins out there that will require configuration or specialized installation methods, so you'll need on the plugin's information page to find out how to do so.
You can download plugins using these sites:
Always make sure to download the latest version of plugins that support your particular version of CraftBukkit/Spigot. If the plugin has not been updated for your version, it is highly recommended that you do not use it.
1. First, make sure your server is running with CraftBukkit/Spigot. To set your server to use CraftBukkit/Spigot, go to your server management page in the control panel and select a CraftBukkit or Spigot build from the "JAR File" drop down menu. You can also upload your own server JAR file if you want to use a different version: https://www.beastnode.com/portal/knowledgebase/72/Uploading-a-Custom-JAR-File.html
2. Then make sure your plugin is compatible with the chosen version of CraftBukkit/Spigot. You can find out what build of CraftBukkit/Spigot you're using by typing /version (in game as op) or 'version' in the console.
3. Connect to your server directory through the control panel's File Manager or an FTP client (guide below) such as FileZilla to access your server and upload your files.
4. After you have connected, simply upload the plugin file (.jar) into the /plugins folder and restart your server for it to install and generate the data/config folder for the plugin (also in the /plugins directory). Please note that '/reload' should never be used - always restart the server fully when adding/removing/updating plugins.
Notes: Some plugins don't generate a config/data folder. Some plugins come in a .zip file and inside the contents are coupled with a folder and the .jar file. Upload them both to the plugins folder.
Guide: How to use FTP to access your files
Plugins can very easily be disabled/deleted through the FTP file access.
To remove a plugin completely, if you no longer use/need it, you simply need to delete the [plugin name].jar file, and the folder with that plugins name, from the /plugins directory and restart the server!
To disable a plugin - meaning the data is still there it just won't be active on the server - rename the plugins [plugin name].jar file(s) to [plugin name].jar.disabled and restart the server. Doing so will mean that plugin is not loaded on the server, but it's data is still present in the files and it can be reactivated at any time.
To re-enable the plugin put the file name(s) back to just [plugin name].jar and restart the server again!