The template of the bug report depends on the bug tracking system you use. Usually the bug report should contain the following fields:
- unique identifier
- build version
- step to reproduction
- reproduction (always, sometimes)
As soon as a bug has been found, it must be reported because you can forget about the bug. Do not prepare the report on a paper (you can lose it) and log the report to the bug tracking system. These are simple rules but many testers neglect them and many bugs have been left unfixed.