When I started learning Roku development I had a luck of working with an awesome senior developer who already had a lot of experience with the BrightScript and SceneGraph.
Everything that I wanted to learn I could learn from him(and I still do).
Unfortunately a lot of developers don't have this opportunity so I wanted to share few links that could help them learn Roku Development.

1.
For learning BrightScript start from codingalien.com and the Roku documentation.
Some information on the codingalien.com page is outdated so the good practice would be to compare it to the Roku documentation(You will also learn by doing this).
UPDATE:
After you get some grasp of BrightScript head over to learnroku/crash-course, this is for now maybe the best source of learning regarding ROKU development for beginners.

2.
SceneGraph, in short, is data structure that Roku uses to manage rendering of the application screens. After learning BrightScript this is where you should go next.

3.
When you get familiar enough with BrightScript and SceneGraph, play a little with the Roku examples from here. You can also check my github for some ROKU examples.

IMPORTANT UPDATE:
ROKU has created a Youtube channel and a ROKU channel with tutorials on for the new ROKU developers. This is really good starting point:
ROKU Youtube Tutorial channel
ROKU Course

VARIOUS TOPICS:
1.Roku Like A Hurricane - Short tips and tricks and nicely explained "how to" blog posts.

ADVANCED TOPICS:
1.
After you learned how to develop ROKU channels, You would need to learn how can you do unit testing on ROKU.
Thanks to George Cook and his great work on unit testing framework called rooibos that will be a lot easier then before.
You can check his unit testing framework, with great documentations and video tutorials here: rooibos.

TOOLS:
1.BrightScript 2D API Emulator - The ROKU emulator for 2d games is still a "new born" and in Alpha stage but with the great potential thanks to the awesome work of Marcelo Lv Cabral . Side Note: It's only for 2D API so it can't be used as a regular emulator for all things ROKU.
2.Roku Atom and VSCode Linter developed by WillowTree Aps

You can always ask for help from stackoverflow community and of course from the Roku Dev Forum.

Notable developers from the StackOverflow Roku Developers Community:
1.Nas Banov
2.Eugene Smoliy
3.alphablender
4.TheEndless

Hope I managed to help.
Have a great day.:-)