#include <stdio.h>
#include <stdlib.h>

int main(void)
        double a = 0.0;
        double b = 0.0;
        scanf("%lf", &a);
        printf("\nThe first number is %f", a);
    return EXIT_SUCCESS;

The question is : how can the program continually re-prompt
I mean when i enter the number, and then the number i entered will show on the second line.
After, I want to clear the program and let me enter the number again.

That's exactly the purpose of loops. There are three types of loops
1. for-next
2. while
3. do-while

To answer your question, all you have to do is put one of those three around lines 7-11, enclosing them in { and }. Look up loops in your textbook for further explanation.