I'm going to help you with some of them.
How can I ensure that the pages accessing is secure enough for data
transmission , sql injection and several other attacks.
For data transmission you should use HTTPS, avoiding attacks between client and server.
Security concern should be addressed when developing the application, choosing a programming language or framework that is capable of implementing CSRF protection, sanitizing HTTP request data avoiding SQL Injection and XSS, and forcing you to use the best practices.
You should be aware that security need to be envolved in all steps when developing your application.
How can I ensure the page which am working on is the original page?
You could Hash the page, or use some type of version control(when developing).
Sometimes the pages get auto refreshed if I am idle for few minutes,
in this situation can someone do by pass-by sessions ?
No, this is most likely implemented by the application to keep the page updated, using HTTP-EQUIV REFRESH.
How can I ensure that the form submitted to the server is proper and
no one has altered it?
Again, you should use HTTPS to avoid any concern with tampering or sniifing between client and server communication. Plus, like mentioned before, CSRF protection would avoid having this form submitted without user approval or knowledge about the content.