The simplest way for you to get started is to download SEO Fast Start
(free) and just follow the directions. You can't go wrong.
Then once you've understood basic on-page SEO, site structure, and internal linking you can begin to work on your link building campaign.
To do that you'll have to find related sites to your niche and then ask them to link back to your site. Follow these rules when making link requests
Then you need to create articles (and lots of them) to build backlinks. For every article you create and submit to a site like eZineArticles you need to social bookmark it. Use a service like Socialmarker
or find a software program that can do this for you such as SENuke
or Bookmarking Demon
For social media marketing backlinks, what you'll need to do is create quality content pages at sites such as HubPages, Squidoo, WetPaint, Knol, Gather, etc. related to your products. These pages should be informative and keyword rich, but do not "keyword stuff". Have these sites link back to your ecommerce website. Don't forget to social bookmark these websites as well.