Help me in Choosing the Field

Actually, it doesn't work like this. You normally start with a certain programming language, say C, Python, Ada, Java and then advance to additional languages and frameworks. .Net isn't an ideal option because it restricts you to Windows (yes, in spite of what Microsoft tell you). Java is a better option because it enables you to work on various platforms. However, be prepared to master both of them, and many other programming languages and tools. Eventually, your employer will have the last word on which tools and languages you'll be using so you need to be prepared to master new technologies fast. That's the nature of this business.