Collect data from web in real time with a java program

I want to practice with my java and I decided to make a program to analyze in real time some bets in diferent sportsbets webs. I need to track all the changes of the prices of some fields in the web to display them in my program. I was thinking in reading the html of the web to find the field that I need but once I have this I don´t know how to track all the changes of that field with my java program. Someone could orient me a little bit about how could I make this?