How do I enable CI error log

[eluser]datactrl[/eluser]
Thank you Research Assistant. I spent a while searching from internet about internal server error. Have done some fix on apache conf files. Such as enable rewirte_module, change <directory>. Unfortuntely, my problem is still there! I tried a every simple Class just echo a word and it works. Is that means CI stating initialing is ok? Have any idea about this problem? Please advise. Thank you!