If your design is for spherical top keycaps, then Signature Plastics is your
only option. If not, then GMK seems to be the most common choice.
As for tools, I recommend becoming as proficient with Adobe Illustrator as you can. Especially if you plan to design novelty legends. Moreover, SP likes it when you can deliver your design to them with their Illustrator template. I don't know what GMK prefers, but I'm sure they like Illustrator files too. I've found that there are far fewer opportunities for scale errors when you give them proper vector graphics files (and no, SVG does not count).
For presentation, you can use
www.keyboard-layout-editor.com, but that is really only sufficient for getting the initial concept across in order to begin getting community feedback. Almost immediately people will start clamoring for photorealistic renders, particularly if they like your idea, so be prepared to start uploading pretty 3D renders as soon as you can.