Below sample bug/defect report will give you exact idea of how to report a bug in bug tracking tool.

Here is the example scenario that caused a bug:

Lets assume in your application under test you want to create a new user with user information, for that you need to logon into the application and navigate to USERS menu > New User, then enter all the details in the ‘User form’ like, First Name, Last Name, Age, Address, Phone etc. Once you enter all these information, you need to click on ‘SAVE’ button in order to save the user. Now you can see a success message saying, “New User has been created successfully”.

Here is the sample bug report for above mentioned example:
(Note that some ‘bug report’ fields might differ depending on your bug tracking system)

SAMPLE BUG REPORT:

Bug Name: Application crash on clicking the SAVE button while creating a new user.
Bug ID: (It will be automatically created by the BUG Tracking tool once you save this bug)
Area Path: USERS menu > New Users
Build Number: Version Number 5.0.1
Severity: HIGH (High/Medium/Low) or 1
Priority: HIGH (High/Medium/Low) or 1
Assigned to: Developer-X
Reported By: Your Name
Reported On: Date
Reason: Defect
Status: New/Open/Active (Depends on the Tool you are using)
Environment: Windows 2003/SQL Server 2005

Source: http://www.softwaretestinghelp.com/sample-bug-report/