If my site structure is like below
MySite (SiteName)
--- HomePage (Page)
--- ProductPage (Page)
Under siteadmin console, while creating site MySite should I use "New Page.." Or "New Site.." ? When to use which option?
"New site" option is used to create a site from an existing Blueprint. Blueprint is part of AEM's multisite manager (MSM) functionality. A blueprint is used to create multiple sites with common structure , for example MySite in English, French, Spanish and so on.
A Blueprint is created from an existing site. A site is a collection of pages, so you will be using "New Page" option for creating the site initially.
So if you don't have "MySite" ready as of yet, you'll be using "New page" option. Once "MySite" is ready and you want to create copies of "MySite" for different languages and countries to enforce a common structure, you will have to use "New Site" option.
You can read more about AEM's MSM capabilities here :
http://dev.day.com/docs/v5_2/html-resources/cq5_guide_power_user/ch13s04.html
http://dev.day.com/docs/en/cq/current/administering/multi_site_manager.html