Consider the program:

#include<stdio.h>

int main()
{
	int a ;
	
	//input age
	printf("Enter the age of the person: ");
	scanf("%d",&a);

	//check voting eligibility
	if (a>=18)
	{
		printf("Eigibal for voting");
	}
	else
	{
		printf("Not eligibal for voting\n");
	}	

	return 0;
}

Output

First run:
Enter the age of the person: 21
Eigibal for voting

Second run:
Enter the age of the person: 15
Not eligibal for voting

It's a simple program and I hope your concept of checking a condition may clear, if there is any mistake then write your comment in the comment box.