Have a record open on the page you want to set the default tab.
Click the Setup button and choose Edit Page. Find the section that contains the tab headings and select it.
Click the Add Tab By default the Details tab will be added.
Click on the Tab in the list and choose the Label from the list. Click the Done
**Only the tab has been added the page. You can now click and drag any object from the list on the right (e.g., report chart, related list, etc.)
Click on Save and Activate to save the changes.
Click on Back to return to Salesforce with the new default tab.