what main features of C are to be learnt well before starting with C++?
Plz reply as soon an possible...
C and C++ are different languages. You don't need to learn any part of C before starting with C++.
I do agree that C and C++ are two distinct languages and, in general, you can just learn one (C++) directly. It's not like C is a stepping stone to learning C++. If you want to learn C++, then you don't need to learn C first, that's just a waste of time.
However, most books and courses will teach some aspects that are more commonly encountered in C (for a lack of a better, standard option), as a way to help understand the "low-level issues", which can be good only if you are doing fairly low-level programming in C++ too. Generally, I think it is also good to have some comparative knowledge of C/C++, mostly to understand why C++ standard libraries a generally considered better (but that's something to pick up along the way, it doesn't require that you pick up a C programming textbook).
Also, because C and C++ are very interoperable, and because C standard libraries are also part of C++ standard libraries, it can certainly come in handy to have some practical knowledge of C. I mean, libraries which are a mix of C and C++ source file are common-place in many settings. And some environments are restricted to C, or at least, to C standard libraries.
Knowing C is certainly an asset in some practical settings, but it's not a requirement for learning C++, and if you are going to learn some C before you get into learning C++, don't spend to much time doing so, move to C++ quickly.