How to overcome the above defined error which occcured while creating a new record?
This simply says the data storage allocated for your organization has been exceeded. You have to delete records and free up some space or purchase additional space. You can see your available data storage by navigating into Setup > System Overview. And you can monitor your usage by navigating into Setup > Administration Setup > Data Management > Storage Usage
If you can confirm some records to be deleted, Mass delete record feature will helpful for you. You can find it at Data Management > Mass Delete Records under Administration Setup (You should have required permission). There maybe lots of Task and Event records hopefully where you no need. You can delete them and free up some storage.
As you might have know that Salesforce work on multitent architecture which put some governer limit across the system. Data storage is one of them. it depends on per User License in different Salesforce edition.
Please find below table with different edition.
Storage is divided into two categories: file storage and data storage. File storage includes files in attachments, the Documents tab, the Files tab, the File field, Salesforce CRM Content, Chatter (including user photos), and Site.com assets. Data storage includes the following:
Accounts Article types (format: “[Article Type Name]”)
Article type translations (format: “[Article Type Name] Version”)
Email messages Events
Quote Template Rich Text Data Solutions Tags: Unique tags Tasks
Go to Setup-> Data management-> Storage Usages and check which object is using most space.. If you think that any object which is not useful to you and have most of the space you can delete those records but if you need you can purchase extra data space from Salesforce.
You can safely and easily mass delete data from the org to return the org to a state of being under the storage limit as well as maintain that status going forward with Storage Helper. Storage Helper is a native Salesforce app that can mass delete data from any object and also has built in options to backup the records it deletes as well as capability to search and restore records just in case. It will send you email alerts when you are nearing your storage threshold so that you don't get this error again. It can also be setup to automatically run after a Salesforce Data Export or on a scheduled basis.