Tips on using Grids

Hi, I am in my 3rd year of my GD Bachelors course at a University in England. This may seem like a stupid and dumb question but what are some essential rules and tips on using grids?

I've always been stronger at the image making, video and illustration side of Graphic design, but wanted to improve on my layouts whilst making books, publications and other forms of media and print.

How does an experienced Graphic Designer know what grid to use with a specific paper size, different font sizes and that sort of stuff. If he chooses a rectangular grid, how do you decide how many columns and rows you have, what gutter/margin size u pick, and how does that change based on whether youre working with just text, or headings and images as well?