Create a macro using data enterd in a specific cell to filter a worksheet column

I am trying to write a macro to be usesd to filter a specific column using the data entered into a specific cell from the user. Example: cell "A3" would be the cell that a user would type in a Site Number (i.e. "FL1234") and Site Name is Column A on a worksheet. I would like this macro to take the value entered into cell "A3" and then filter column A. I did write a macro, but it only works the first time and then it keeps saving and using the data entered in it the first time and will not clear if I enter in anything new into cell "A3" -- probably because I did a recorded macor instead of actually writing it to try to save time.

A3 = "variable" (this is the cell that the user will enter the search/filter criteria into
Column A = filtered search target (this is the column I need the cell "A3" to filter into

thanks.