While we are more than willing to help those that are stuck, that doesn't mean that we are here to do it all for you! We can't do all the work, you are either getting paid for this, or it's part of your grades and it wouldn't be at all fair for us to do it all for you.
So we need you to do the work, and we will help you when you get stuck. That doesn't mean we will give you a step by step solution you can hand in!
Start by explaining where you are at the moment, and what the next step in the process is. Then tell us what you have tried to get that next step working, and what happened when you did.
Start by thinking about how you would do it on paper: you'd count the number of characters in the string, then loop round that many times.
The first time, you'd print 0 "-" characters and all the ones in the string.
The second time, you'd print 1 "-" character and all but 1 of the ones in the string.
The third time, you'd print 2 "-" characters and all but 2 of the ones in the string.
And so on.
That tells you what you need to write. Me? I'd write a method that takes a string and an integer, prints that number of "-"'s and the right number of string characters. Then I'd call that inside my loop.
If you are having problems getting started at all, then this may help: How to Write Code to Solve a Problem, A Beginner's Guide