Hey,
Unfortunately the latest version of client v0.3 isn’t yet documented, we are in the process of create documentation.
I guess you have found this Issue which contains a draft installation guide of client in rpi3.
If you have any comment or question on this guide, let me know. It is indeed complex as it is more a guide for people who want to develop the client rather than for simple users, however it should work fine.
About the api key you need, you can sign up in network-dev there you will be able to create your station and by clicking on the top right corner on your username and then on “My Profile” you will see your profile page. There by clicking on the top right corner the API key button you will retrieve your key.
After testing in dev network that everything works fine, you will be able to join the production network.