Making A Registration Form

I have a few textbox to be filled in the form.

Eg:
Fistname: (textbox)
Lastname: (textbox)

I would like to do some checking on user input when he/she submit the form.

Eg:
Fistname: BBB 'error
Lastname: bbb 'correct

If there happens to be error on the user input (eg. user input in uppercase), it will go back to the form and list the errors.

For those error input, their respective textbox will be cleared for user to re-enter again. For those which is correct, the user input is displayed in the textbox (remained unchange). Code: