How do I edited X keyboard layout?

I want to edit my keyboard layout to something that'll be more comfortable than what's printed on my keys. I already did that on Windows, and I want to do it on Linux. I believe that what I need to edit are X keyboard layouts (not sure of what should I do to do that in consoles, maybe create a new type of terminal?). So my question is: are there any keyboard layout editors for X, or what files do I have to edit with what documentation?

man xmodmap