Build it up bit by bit.

Start with a for loop that displays a number of asterisks. Make it so that you can set the number, e.g.
Code:
int num=10; // this determines the number we print
for ( /* ??? but reference num */)
{
  /* ??? */
}
If that's too difficult then just write a loop that displays a fixed number of asterisks, say 5. Then replace the 5's with num and you can experiment by changing the value of num a few times to make sure it works.

Then enclose that lot in a loop that varies the value of num. By the looks of it each line is one asterisk longer than the previous so that should be easy.