os mountain lion

hi i am new to this as i just bought my 1st ever macbook air 13.3 iam entitled to mountain lion for free when it is availible ' i have been told that i have 30 days to request it but i am not sure how to do this......i did ask an apple repbut she was not sure i am just a wee bit concerned that i will not request it in time.. thanks in advance :-)