how best to use Treehouse for career change/development?

Depending on where you live, it might be worth doing a little research and seeing what IT jobs are going locally, so you can see what employers are looking for. Most jobs I've seen where I am want a degree, so I'm dong one part-time, for example. Certification might be worth investigating.

Building a portfolio of work is a good idea as well. It means you've got a body of work to show people what you can do.

Finally, do other courses as well. You might want to focus on one specific role to begin with (web design, so learning HTML and CSS to begin with for example), but then you might want to learn Javascript, PHP etc, to expand on those skills. As with anywhere, the more skills you have, the more employable you are.

You should check out some of the workshops teamtreehouse provides on getting hired in the industry. Also go here. https://teamtreehouse.com/career-resources

The advice given by Greg and Colton is really good, so I won't repeat it. I'll just add to it.

Many front-end jobs will also require skills in design - logo creation, wire-frames / sketches, photo editing. It's also becoming much more common, from my own job searching, that many positions are looking for 'full-stack' developers (that means front-end and back-end; client side and server side).

I recommend watching Web Development Career Advice by LearnCode.academy. This guy breaks down all the languages and technologies you'll need to become familiar with on your path as a developer - in both front-end and back-end roles.