After recently starting to earn a salary, when do I need to pay zakat?

I have few doubts in mind regarding Zakat. 1) I have started earning Salary since February 2017 and it's Ramadan now, so I need to know when do I need to pay Zakat for the salary which is getting accumulated since February 2017? 2) Also I would like to know that Zakat is paid on wealth above nisab. So is the Zakat paid on the wealth which is only above nisab or the whole wealth including Nisab?