Not sure where to start? See Get started with .NET in 10 minutes.
Before installing .NET, you'll need to register the Microsoft key, register the product repository, and install required dependencies. This only needs to be done once per machine.
Open a command prompt and run the following commands:
wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb sudo dpkg -i packages-microsoft-prod.deb
Update the products available for installation, then install the .NET SDK.
In your command prompt, run the following commands:
sudo apt-get install apt-transport-https sudo apt-get update sudo apt-get install dotnet-sdk-2.1