.NET: Microsoft Technologies based on the .NET software framework. Machine learning: A type of artificial intelligence focused on enabling computers to use observed data to evolve new behaviors that have not been explicitly programmed.
you first need to know how to build model in PyTorch. here is a beginners guide (it assumes you know the basics of Neural Networks and tensors):
https://sebastianraschka.com/teaching/pytorch-1h/
maybe you want a higher level tool like ml.net:
https://dotnet.microsoft.com/en-us/apps/ai/ml-dotnet/model-builder