First check that the issue has not already been reported.
If reporting a bug, are you sure it really is a bug in JMeter, not just a misunderstanding of how JMeter works?
If you have not already done so, you need to register an account first, using the "New Account" link at the top of the
main Bugzilla page:
https://issues.apache.org/bugzilla/
.
Make sure you read and understand the information on the account creation page before signing up.
Once logged in, click "File a bug" and select JMeter from the list.