We have one requirement,Client is asking to convert a task to a case.
How we can achieve this ,pls help in this regards..
There are several methods you can try, depending on your requirements.
If you are simply mapping field values from the Task to the new Case object and not adding or changing any additional information you could create a custom button on the Task to create the Case. An example of this can be found here.
If the user needs to customize the information being added to the Case, then you would want to add a custom button to the Task object that directs the User to a custom Visualforce page where they can customize and create the Case.
If this needs to happen automatically without the user's input, then you could use a Trigger to create the Case using the Task fields when certain conditions arise.
You could create a Flow that, when a user clicks on a button on the task, a case is created. Here's a DreamForce session video that will probably help.