Where to start?

I like using the 'Tracks'. It puts everything in order so you don't miss a prerequisite. The harder part I think is picking a track, but that depends on your goals.

You can go to whatever course you are interested in and just start it, however if you are new to Treehouse and Java in general and wish to have a guided learning experience then I recommend joining the Learn Java track.

Tracks are essentially just collections of courses that have been handpicked to guide you though learning some language or topic. By joining the Java track you will start with the Java Basics course but once you are done with that you will automatically move on to the next Java course that is designed to be viewed after the basics course, and so on.

There are a ton of other tracks available. So if you have some other programming topics you are interested in I'd recommend heading over to the tracks area of Treehouse and joining whatever tracks that sound interesting to you.

In addition to the great advice in the posts above I would highly recommend you try the Front End Development track first. You'll inevitably deal with JavaScript in that, not Java but I think it's essential for getting some grounding in CSS. You'll want to learn how to style your apps and whereever you end up in the tech and/or development world you'll need to know CSS.

Good luck. :)