Uploading and wordpress arent really connected, except that sometimes one must "upload" to extend the functionality of wordpress. And some folks even have to upload WordPress, it appears you installed it via Plesk or Cpanel.. or something similar.
Once you have an ftp client installed, it's a matter of connecting to your ftp with your client. The interfaces of ftp clients are going to vary from client to client, they do not all have the same buttons, features, etc..
Your ftp login and password is typically given to you by your host at the time you initially set up your hosting.
Once you have ftp'd in you will see a directory structure not unlike what you would see on your own computer in, say for instance, the "explorer" (start/run/explorer) screen in Windows.
Every WordPress installation comes with a plugin directory. It's located here:
Root of WordPress Install/
wp-content/
wp-plugins/
Plugins get uploaded to the wp-plugins directory.