In DNN, a web site is broken up into pages, just like with any other site. Within each page there are a number of Panes. Panes are aligned into a Grid. This grid is what makes multi-column layouts possible... things like a left column for navigation links. It is important to remember that the Grid is always 12 units wide. Every row in the grid will have panes set widths, and those widths always add up to 12 units. If you add content to one pane in a row, you should add content to all panes in that row. You can use as many rows as you need, but each row has a different layout. Most pages will only use one row in the grid. There is a handy grid reference here.
Panes do not host content directly. Instead, to add content to a pane you must first add a Module. There are a number of different kinds of modules available, but 95% of the time or more you will just use an HTML/Text module. This module provides you with an easy-to-use text editor, similar to Microsoft Word, that you can use to format a page. As a side note, it's worth mentioning here that it is very poor style to try to fit an entire page into one module. You will have much more attractive and readable pages if you can break up your page into groups, with each group in it's own module. There are also some advanced modules available. You can find out more about them by looking under "Modules" on the menu.
Modules also have Containers. Containers are the decoration and formatting that appear around and behind your content. There are a number of different containers available to you, but several of them have a specific assigned purpose, and you should only use those containers for that purpose.