Step 1: As an Administrator, go to the Setup link at the top of the screen
Step 2: Click on the “Company Information” link under “Administration Setup” and “Company Profile” in the side navigation
Step 3: Your API Request usage is on the Organization Detail page
Note: if you don’t see the API Requests number, it could be because your Salesforce instance doesn’t have the API enabled. Only Salesforce Enterprise Edition includes the API by default. Salesforce Group and Professional Edition don’t include the API, although you can pay to get API access with Professional Edition.
System Administrators can also access an Administrative Report on API Usage in the “Reports” tab to see if there are any other applications that are using your Salesforce API calls
This blog information is very useful to learn about API usage in salesforce.