Help! the steps required and technical to build the following addon.

Hello everyone! i'am a new member
I want to build an addon with basic functions
 - get text (A) from page ( facebook.com/... , youtube.com etc ) like comments.
 - call the web service function process this text (A) and return another text (B)
 - replace (A) by (B) and show it for user

this is basic ideal of my addon. but i don't know the steps required and technical to build it. please help me clarify it. thanks