In magento, the html on any page comes from many different phtml files. So if you want to make a change in any page, you need to first know from where that html code is generated. To know this, you need to enable template path hints from magento.
To do this go to System -> Configuration
In the top left there is a box named “Current Configuration Scope: “ which has a drop down.
From that drop down, choose your store view name. By default it is last option in the drop down i.e Default Store View option. After this go to Advanced -> Developer->Debug
And here make Template Path Hints = Yes
After this save the configuration.
Then when you open frontend, you will see which should all the pthml file used.