Please tell me how to break/Secure the security of any login page.

I want to know the security issues associated with any login page.
and how can we break the security using SQL injections.
Please give me some example queries and descriptions.

and also let me know how a programmer can protect their site from being break/hacked...

Thanks in advance for reading this question !

Please mail me or reply me with your answer's I will be very much thankful to all of you.
😁