Building layout with Grid System
This topic is sufficiently covered in multiple tutorials. You can also look into alternative grid system implementation such as that in Bootstrap, 960.gs, Blueprint, etc.
Agile Toolkit grid can be configured to work with either a fixed layout or flexible layout. @gridColumnWidth variable in atk-main.less can be specified in either percent or pixels.
Many Agile Toolkit add-ons will rely on standards CSS classes so even if you are not planning to use them, it's usually a good idea to leave them enabled.