How can I use Eclipse or any other IDE to write the code for plugins? (Or code from the command line?)
Do I need to download Jive files to get started?
Do I need to use Maven, starting here?
Do I need to do that in order to compile a widget and JAR it? Can I just package together my own JAR and compile it from the command line? Especially if I'm doing something as simple as this Hello World tutorial?
You will need to configure and install Maven for Eclipse to access Jive's repository to use Jive's API. https://community.jivesoftware.com/docs/DOC-3547should provide all the details you need to get Maven installed and configured properly. You will not be able to compile any plugins/widgets until you finish this step because your development depends on Jive's API, which will all be available through the Maven repository.