Minecraft is a client-server system, so you’ll need both parts: the desktop graphical client that you use to play the game, and the server process that you connect to, where we’ll add plugins.
You probably have this part already, but if not, download the Minecraft installer for your computer from http://minecraft.net. Follow the installer instructions to install it, but don’t run it just yet.
When you play Minecraft, this “client” is the application that you run. It connects to your paid account at http://minecraft.net.
There are also clients for Apple’s iOS and Android devices, including smartphones, but they are a dead end. As I’m writing this, they don’t connect ...