How to show unity in c# windows form?

You can achieve that type of implementation with a custom project solution that u can create with Visual Studio named "App Windows Forms" where u can drag items like buttons and images and code their behaviour with events.

On Unity you can use Canvas to create UI Elements (buttons,image) and create the same thing but not with forms but with scenes.

If u want to use forms with unity I find this guide on youtube: https://www.youtube.com/watch?v=Kor22uBSYbs

Hope I helped u some way!

Since you only asked where to start, I would suggest you start here.