OK, so the first step is to make the variables local to main, so change:
Code:
int value,rem,bill_50,bill_20,bill_10;
void dispenser();

int main(void)
{
to:
Code:
void dispenser();

int main(void)
{
    int value,rem,bill_50,bill_20,bill_10;
Then dispenser() needs redefining to take pointers to these as parameters. You should have something somewhere in your course notes that shows an example of a function that takes pointer arguments.

Then within dispenser, change the references to these variables from simple "value" references to pointer references, i.e. "*value" (assuming you keep the same names). For example:
Code:
if(*value>=50)