Hi @Kashmitha Madushan,
At the beginning you could follow video tutorial to make basic Asp.Net core MVC website with using EntityframeworkCore to operate the database. Such as this tutorial. Then you will have a basic full stack view of c# back-end developing.
After that I recommend you follow this popular roadmap to improve and practice your skills to master every section about back-end developing.
https://github.com/milanm/DotNet-Developer-Roadmap/blob/main/NET%20Roadmap.png
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