a) Yes, you put the Roku in developer mode and as you write code you zip it up and upload to the Roku.
This is how you test and debug it. More specific instructions are in the docs provided with the sdk.
b) Your computer would be the workstation, the text editor could be notepad or something similiar.
You can google 'programmers editors' find many free ones.
I use Scite.
Google putty if your on windows for a free terminal application.
Mac and Linux have one installed. See OS docs to find how to access them.
c) nope, that should do it.