12 grid column system

Hey guys, I am very new to UX/UI web design and Im suppose to work with the 12 column grid system. However Im still not quite sure how does it work, for example if Im working with the screen size 768x1024 (standard for tablets), and one column width is 60px, it does not fit into the size for 12 columns. How am I suppose to maintain the 12 column grid rule but still keep the same size of 768x1024 px?

Use fluid margins.

I had a much longer post and it’s gone. … sigh.