If you are using the netinst disk, you will be prompted to use an online repository (mirror) that will most likely be preselected based on your country selections. You can choose one if you configured your network interfaces correctly, or you can skip it at any time (a nice trick is to use Esc to go to the upper menu on any function) and just install the basics that come with the netinst disk.
If you successfully selected and have access to a public Internet repository (archive), the installer will prompt you about installing tasks, which are basically families of packages that are precategorized for simplicity. Netinst doesn't actually carry many tasks because the disk itself does not carry many packages; however, just for reference, the Web server task installs Apache while the Database task installs PostgreSQL for you. Otherwise you'll be OK with the Standard System Utilities task.
Also, you will be asked to opt in to popcon, an anonymous survey where the packages you use and install are sent to Debian anonymously for statistical purposes. We suggest you choose the default (opt out) especially to avoid outgoing traffic from your production servers, but if you want to help the Debian Project know better the profiles of installed computers all over the world, the egress is just a few kilobytes a day.
You are almost set. The final question of any Debian installation is whether you want to install the GRUB boot loader or not (you might be using your own, or a particular architecture) which you do, so just press Enter. Installation will wrap up and reboot (the disk may be ejected, if not, eject it yourself), and the server will boot into your new system after a brief wait on the GRUB menu (you can press Enter to skip). The init system will kick in and you will be facing a login prompt with your hostname after a few seconds. Type in root and your password, and you will be facing a brand-new dash prompt. Welcome!