How to move from static php to dynamic content

Hi all,

I have been coding php/sql/html for about 15 years now as a hobby. I am fairly proficient in doing the basics, what I am seeing now is more and more dynamic content in pages. Looking on advice on how to progress into doing some of the following stuff

1) Form fields
- right now I use html forms with a submit button and a php script to catch the submission process it and put it in a database
- the trend now seems to be, user enters something into a field, moves the focus away from that field which triggers the field to be saved

2) Events after a page load is complete
- I have a script that checks for new messages into a users mailbox, everytime you load a page it checks and shows a message if there is anything new
- facebook etc will popup instantly at anytime when you receive a new message regardless of what page you are on etc


Basically i am asking where to start? and what language/code should I learn to be able to do this stuff?

thanks in advance!