In the case of Grassblades, I'd been reading far too much Blade of the Immortal, Vagabond, Usagi Yojimbo, Bleach, Lone Wolf and Cub and Hellboy - and watching too many EastAsian costume-dramas. So the thought of wanting to draw a comic about samurai was sparked in my head.
It sort of grew from there. However, I've been drawing comics for years - Grassblades is just my first long-format webcomic that got to be more than 2 chapters or so. So I have prior experience making comics - and I've been drawing for most of my life. I draw both traditionally (pen on paper, watercolour, ink, acrylics, etc.) and digitally, but Grassblades is 100% digital - not counting the thumbnails for the pages.
As for your specific questions....
How to come up with ideas: This one varies from person to person, so I can't give you a definitive answer. All I can tell you is to sit down with a pencil and a piece of paper, and start scribbling down ideas. It doesn't have to be huge, or complicated, or even particularly well defined. Grassblades literally began with a drawing of a grumpy dude and the thought "Hey, samurais are cool - I should do something with samurais" - and now it's this giant, complex, intricate thing. Once you've got a bunch of ideas down, read through them and see if any of them fit together. If they don't, rattle them around a bit and see what falls out.
Put smaller ideas together, and they're more likely to become big, inter-connected ideas - a bit like little bits of mercury pooling together until you've got a great big puddle of it.
Say you want to do a scifi-story. Why? What kind of things interest you with the scifi-genre? In my case, the answer is adventure, exploration and discovery, and themes about what makes us human and what doesn't - so when I make a scifi comic, those are the kinds of themes I like to explore. Having settled on that - what kind of characters would you need to make that story happen? Etc., etc.
Of course, you can go the other way around and come up with an interesting character and then craft the plot around them - it's all up to you.
How to plan a comic Again, this varies. Some people like to write pages and pages of script - others like making stuff up on the fly. I'm somewhere in the middle, as I work mainly with a rough outline of events, and then only really write the comic (dialogue, etc.) at the same time that I plan my page-layouts. I need to see what the comic will look like before I can settle 100% on the dialogue.
I've got an outline for the entire comic, but I only storyboard - i.e: page-layouts and dialogue - one chapter at the time. This leaves me space to tweak and adjust things as I go.
How to learn to draw digitally Pick a tablet, pick a software, start practising. Every drawing-program is different, and has its own challenges and limitationsI do most of my drawing in Manga Studio 5, but there are people who draw in Photoshop, Paint Tool Sai, GIMP, etc.,. It is possible to draw digital art with a mouse instead of a tablet, but it's a LOT harder, and can be hell on your wrist.
I use Manga Studio, as I said, as well as doing certain things in Paint Tool Sai, and I draw with an Wacom Cintiq 21UX, but there are plenty of cheaper options that are perfectly good for digital art.