Magento SEO - Onpage

Hello Guys, I'm Magento website developer and recentaly I have created an eCommerce website in Magento CMS. Now my client want to run SEO campaign for their keywords. I have some little questions to make Magento store more SEO friendly.

How many H2 tags should add in internal pages?
How to add Google webmaster tool for my Magento store?
How can index all pages in Google?
Is HTML sitemap effect my SEO?