How much tax you pay when you sell a share is generally determined by:
The difference in price between when you bought and sold the share
The length of time between buying and selling the share
For specifics of how much tax you would or will be paying, please consult an accountant or read through information available online from the ATO.
In many cases, for individuals, if your TFN is registered with Pearler then the ATO will help you calculate through the government's tax apps.