How do you want to set up your site?
Install October without any plugins or themes. This is a good option to choose for custom building.
You'll need to code your site in HTML / CSS.
Pick from a pre-built site that fits a general purpose and you can customize later.
For example: A blog site, a portfolio site.
If you've set up a project at the OctoberCMS website you can enter it here.
This option can be used to define plugins and themes manually.