In the Tasks tab of the project's primary horizontal menu, click on the New Task button. Enter a task title and press return/enter or click on the Save button to save a task. You can also click on Save & New button to create more tasks.
Users will need view and edit permissions in tasks to perform this action. Depending on the user's project permissions, they may be able to view all the project tasks or just a subset of them. Watchers specifically added to the task will be able to view it.