i have an employee who has worked for me less than 40 hrs. over a period of two weeks, do i have to give one week notice or can i let her go with just written notice

Under the Employment Standards Act, you are not statutorily required to give any notice (or pay) to an employee who has been with you for less than 3 months. However, unless you have a contract with the employee that says that they are entitled to nothing if they are terminated within 3 months of starting, then you will owe the employee some notice or pay under the common law. You can provide working notice or pay in lieu of notice. I would strongly suggest that you contact a lawyer to discuss how much notice should be provided and, for future reference, how to draft contracts such that you are not in this position in the future.