An ad unit is a slot on your website that is integrated with a tag. You have several options when it comes to layout: Either create a block for a single ad or play out multiple ads in one ad unit. Here (link) you can find out everything else you should consider when creating the layout.
First go to the "Ad management" of your publisher account and then to "Overview". Now use the plus symbol to select the website for which you want to create an ad unit. Click on the button "Add ad unit" and you will be redirected to create your ad unit. First give the ad unit a meaningful name - it is common to use the format as the name, e.g. 300x600. This will make it easier for you to keep track of all the ad units you have created on your site.
Here we continue with the layout design of your ad unit.