Checking if a Number is Entered?

I'm trying to check if a number is entered in a textbox. Yes, I know how to use IsNumeric, but that's not what I need. Let's say someone gives you a form that has something like First Name with a textbox next to it. Then they enter something along the lines of Harry1234. I don't want them to be able to enter numbers. How can I go about verifying that they didn't?

Please explain as simply as possible, I'm still learning. Thanks for your help.