When you install a plugin on server you must check before the version that you install.
If you install a plugin in 1.7.10 on a 1.8.8 server it will not work.
Firstly, if you want to have plugins on server you need one of this following packs installed on your server:
1. You need to have a pack that accepts plugins. Here I have Craftbukkit 1.8.8. Read how to install a pack => here.
2. Go to "Plugins" panel to search a plugin to install.
3. Search "essentials" plugin in search tab.
4. You can click on plugin or get more information with "info" button.
5. The most important moment, you need to check the version (remember I have Craftbukkit 1.8.8) but official Essentials version is 1.7.9. I'll install it as well to show you what happens.
6. Click on install button.
7. Go to "Console" to see what happens when we starting the server.
8. RESTART server after each plugin installation (please don't use /reload command, in the long run, it corrupts files and chunks.)
9. Live console indicates that the plugin doesn't installed:
[10:47:59] [Server thread/ERROR]: Error occurred while enabling Essentials vPre188.8.131.52 (Is it up to date?)
To conclude don't install versions that don't match together. Go to "Plugins" panel and remove all plugins for the rest.
Now we are going to upload 1.8 Essentials version from https://hub.spigotmc.org/jenkins/job/Spigot-Essentials/
10. Download "Essentials-2.x-SNAPSHOT.jar" plugin.
11. Connect to your server with FTP access in using Filezilla or Winscp.
12. Drag and drop Essentials plugin into server "plugins" folder from your Windows explorer to Filezilla.
Restart your server and check again the console, if there is no error it's good, but if you have errors check again.
To conclude in our situation we have the good version of the plugin for the server.