Minecraft and Minecraft servers are Java. That's what you'll need for a plugin (I'm assuming you mean an actual plugin, and not a mod for the client, even though they're both Java). All plugins require a
.jar file to run, and optional configuration files, so you'll need to think about that (config is not needed, but you might want it).
How much experience do you have with:
1. Coding Java?
2. Coding plugins in Java (I'm guessing none?)
3. Running a bukkit/spigot server (with plugins)?
Remember to back up your server files before loading new [versions of] plugins.