how can i perform multipication of two very large numbers(example 10^18 * 10^18) and store them in C language.?

You can do it in the manner you would do by hand - multiplying, carrying, etc using an array of integers, and then store it in a 2d array, or as a string. Some other fast multiplication algorithms are -

(Fast Fourier Transform based multiplication) http://numbers.computation.free.fr/Constants/Algorithms/fft.html

Hope this Helps!

karatsuba will be helpful… thanks @ priyanshu_95