This is a really vague question as we have no idea what you already know.
Do you know C#? If not then learn it first.
Have you ever written a web app before? If not then you should learn the basics of HTML, CSS and building web apps. Mozilla Developer Network has a good set of docs.
Are you building an MVC, Razor Page, Blazor or client-side app (Angular, React, Vue, etc)? MVC/Razor Pages is ASP.NET Core. For client side apps you'll need to go to their tutorial sites.
Do you have any ASP.NET experience? If so then refer to the migration documents for ASP.NET Core.
In all cases Microsoft Learn is a good, free starting point. PluralSight is good if you have a subscription. Googling for tutorials and examples is how I personally prefer to learn.