Cool MeMos give us a nice example. How it should look like if you are starting from zero. But in case you are editing an existing page the things are a bit different. Cause sometimes if you have missed some general <div> Tag you may not be able positioning as you like
It's important to take a look on the hole html to find those tags and see/edit their styling if need so.
It's simple to think that the div tag is a block and then you can insert many blocks into this one. Easy example:
Your main div is with width=300px, and height=300px,
And if You want to insert another block inside this block with bigger size the main block and to stay in left side it's impossible. Unless you edit the height of the main block with different size or set different position:absolute or fixed for the second block.
So it's more complicated if you are trying to edit existing web, you have to pay more attention on hole web. But this is the best way start learning.