What to do when the storage limit is exceeded in salesforce?

As you might have known, Salesforce works on multi-tenant architecture which puts some governor limits across the system. Data storage is one of them. it depends on per User Licence in different Salesforce editions.

Please find the table below with different editions. Regarding storage limits exceeded in salesforce - 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”)
Campaigns
Campaign Members
Cases
Case Teams
Contacts
Contracts
Custom objects
Email messages Events
Forecast items
Google docs
Ideas
Leads
Notes
Opportunities
Opportunity Splits
Orders
Quotes

Quote Template Rich Text Data Solutions Tags: Unique tags Tasks

Go to Setup-> Data management-> Storage Usages and check which object is using the most space.. If you think that any object which is not useful to you and has most of the space you can delete those records but if you need you can purchase extra data space from Salesforce.