Hello @Sudip Bhatt
Start here, What's new in C# 9.
For setting up a project in VS2019, double click on the project file in Solution Explorer and alter as shown below
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>net5.0</TargetFramework>
<LangVersion>9.0</LangVersion>
<RootNamespace>YOUR NAMESPACE</RootNamespace>
</PropertyGroup>
</Project>
Note that when exploring C# 9 many sites still show proposals e.g. using ! for not null for method parameters which in this case didn't make it in but instead rather than ! [NotNull] attribute was introduced.
private string NoNullExample([NotNull] string value) => value.ToLower();
Records seem to attract a good deal of attention so check this out and on the left other C# 9 docs
https://learn.microsoft.com/en-us/dotnet/csharp/tutorials/exploration/records