"there is no rule that one should first learn assembly and then go for C"

True - but Michael has given you his opinion.
Not only has he given you his opinion; he has also given you a detailed (and very well founded) justification of that opinion - both in general and in your specific case.
You came here asking for opinions and you have received a very good one - don't bite the hand that feeds you!

"i have if not much ... knowledge of asm"

Exactly - you have started but not finished the job. Just giving up and switching to 'C' will not help!

"so please tell me how should i start learning C for 8051."

So, you asked this question originally, and didn't like the answer. Simply asking exactly the same question over again will not help!
Unfortunately, this is characteristic of your posts here - Several people have already highlighted it to you.
You keep asking questions, ignoring the advice, and then asking the very same question all over again!