Need help in algorithms?! please!!!?

im doing a computer studies project the subject is about a car part company it basically sells car parts of different brands. i need an algorithm that will keep counting the number of stocks left and alert the employee/user when the stocks are running out. you specify a specific limit so that everytime the user inputs a variable it compares the no# of stock left with the llimit specifed. if u can help me i would be so grateful. thank you:)