Is it necessary to learn C language first to learn Java language, or is it important to know something about C to learn Java?

No there is no necessity of learning c for learning java. but it will be easy fro you to learn c language first and then learn java.

C is a mother of all high level languages so learning C will give you enough understanding of the basics of all high level programming language so you will learn other languages with ease. You can learn java without learning C but the best way is to get the knowledge of C then move to java.