Prompt user for Test Scores -- Need Help

I'm extremely lost on how to do this...

If anyone could help, it would be greatly appreciated.

I need to write an application in c# that continuously prompts a user for test scores until the user enters a sentinel value. A valid score ranges from 0 through 100. When the user enters a valid score, add it to a total; when the user enters an invalid score, display an error message and before the program ends, display the number of scores entered and their average.

The code needs to contain a while loop.

Thanks