To create an Automation, follow the below steps.
1. Click on the gear icon in the top right corner of the page & click on setup.
2. Type process builder in the quick find box & click on the Process Builder.
3. Click on the New button to create a new process that stats when a record changes.
4. A new window gets poped up when you click on New button. You need to give the process name and short description about the process and need to select when a process needs to be started. Click on the Save button.
5. You will land on the page as shown below. Where you need to select what will happen when the process starts.
6. Click on Add Object. and select the targeted object. here e.g Contact.
7. Click on the Save button.
8. After selecting an Object you need to select criteria. Click on the Add Criteria. Give the criteria name and select the Conditions are met as Criteria for Executing Actions. Set the desired Conditions in this case Mobile number is not null and the criteria would be <ANY field> <is changed> <Boolean> <True>
9. After setting the criteria, you need to add the Immediate Action which will get executed right after the conditions are met.
Select the Action Type as Create a Record from the dropdown menu. Give Action Name and Select Record type as Task.
10. After Selecting the Record Type as Task You need to set the field values for the record creation as follows. After setting the field values click on the Save button.
Note: You can either use Related To ID or Name ID
12. Click on the Activate button.
Thus a process of creating a task will be activated.