How to manage usability with multiple http error messages?

1. What are the guidelines for effective error messages?
2. What is the best way to show an error message when a user inputs some wrong data and submits?

What are the guidelines for effective error messages?

Summary: Established wisdom holds that good error messages are polite, precise, and constructive. The Web brings a few new guidelines: Make error messages clearly visible, reduce the work required to fix the problem, and educate users along the way.

What is the best way to show an error message when a user inputs some wrong data and submits?

Error messages should be clearly indicated

And you should be clear where that error has been made. You can do this by: Highlighting the field in question visually, usually with an outline around the input in a bright noticeable colour (usually red) Adding additional icons or visual cues to draw attention to the mistake.