Please this is a C question.

We won't do your homework...

But:

1. Learn how to show messages in the console (printf will do it).
2. Learn to get numbers entered by the user. scanf or similar functions will help you.
3. Learn how to loop in that case a for loop will do it as you know how many numbers have to be asked.
4. Be clever and use only a numeric variable to store the sum and increase that variable inside the loop.

Note: be careful, if you enter big numbers you can suffer from overflow when making the addition.

Now, with all that information, you can start using google to learn how the functions work. Then my recommendation is that you should design the app using a pencial and a paper, and at the end you should start programming.

Divide and conquer! <= this is one of the most useful tips one can give you. It always works and it is the only way to do it.

Good luck!