To create a task, open Tasks window and click on the button on the left side - "Add Task":
As you noticed, there is a field: "Select parent task". You may choose any task available, there is however a special one: Root in case you don't want your new task to be a sub task.
Write description, select priority and estimates if you wish. You may change them later if you wish, just double click on a task or select a task and click Properties from a tool bar on the left.
There is another way to create a task. If you have TODO items in your project, you can put cursor on them and use intention, as it show on the following picture:
Basic actions are available from the tool bar on the left, but there are some more in the context menu.
There are many ways to organize your tasks, here is just one I used to develop Tasks plugin itself:
To track actual time, you have to turn that option in the plugin settings. They are available from IntelliJ IDEA settings.
Tasks tree appearance will be changed a bit, to show estimated and actual time. Also when you complete a task you will be asked
to enter actual time. If you turn this feature off, you won't lose actual time data, it will be just hidden.