So, say you want to set up a 100-pixel left margin and allow about the same size right margin. That goes before all your text. You can also include images and put tables inside your tables if you want to. You've set up two invisible cells that stretch down the length of the page. An even easier method, if all you want to do is center text with broad margins to either size, is set up a table with a single data cell of a set width.
Tables can be very complicated, or very simple.
You will probably want to draw out the layout of your page and plan the table on paper before constructing it. Or - You can put a tiny little table inside a cell to subdivide it. Okay, now we get complicated. A simple table looks like a spreadsheet. Lots of little rows going down the page.