Determine If Directory Exists, Create It If Not

I wanted to create a login system that would have 3 tabs 1 for regular login, 1 for admin login and 1 for account creation. So far so good. It saves the accounts by saving them in a .urs file in the map called /Users/. However when this map does not exist it will show a error. I would want to know what code or so to put into the form_load which will make my program create the map /Users/ if it does not exist. As im not familiar with all the codes and stuff. The only thing i can come up with is

Mkdir "/Users/"

but then ofcourse it would create the folder users everytime i start the program. not i was still "Hello Worlding" a few days ago, so i really am unfamiliar with any sort of high-skilled programming which i probably dont understand.