There are a number of ways to download the source code. Some are described here, an alternative all-inclusive guide is also located at https://www.tgstation13.org/wiki/Downloading_the_source_code
Option 1: Follow this: https://www.tgstation13.org/wiki/Setting_up_git
Option 2: Download the source code as a zip by clicking the ZIP button in the code tab of https://github.com/DaedalusDock/Gameserver (note: this will use a lot of bandwidth if you wish to update and is a lot of hassle if you want to make any changes at all, so it's not recommended.)