Search for "download .net core" and follow the instructions on Microsoft's download page to get the .NET Core SDK. After the SDK has finished installing, open up the Terminal (or PowerShell on Windows) and use the dotnet command line tool (also called a CLI) to make sure everything is working:
dotnet --version
2.1.104
You can get more information about your platform with the --info flag:
dotnet --info
.NET Command Line Tools (2.1.104)
Product Information:
Version: 2.1.104
Commit SHA-1 hash: 48ec687460
Runtime Environment:
OS Name: Mac OS X
OS Version: 10.13
(more details...)
If you see output like the above, you're ready to go!