Your homepage should definitely be the primary focus of your SEO campaign, as with most. Being an e-commerce site, it's going to be easier to first optimize your homepage and your categories. Your homepage for your main goal (iPod, MP3 Players, etc.) and your categories for
what they are.
Code:
Homepage
-- MP3 Players
---- iPod
------ iPod Touch
---- Zune
------ Zune 16GB
-- Accessories
---- MP3 Players
------ iPod
-------- iPod Cover
------ Zune
-------- Zune Adapter
or
Homepage
-- MP3 Players
---- iPod
------ iPod Touch
---- Zune
------ Zune 16GB
---- Accessories
------ iPod
-------- iPod Cover
------ Zune
-------- Zune Adapter
So of course you're going to optimize your homepage for the place to get MP3 PLAYERS and ACCESSORIES.
Then optimize your categories for whatever they are. The IPOD or the ZUNE, ACCESSORIES, etc.
A big part is your navigation, your URLs and your onsite SEO.
Your navigation should always be user friendly first, but allow for SEF as well. Your navigation should dictate your URLs and everything else that identifies a page, category or product such as titles and breadcrumbs.
The navigation above dictates the following URL:
site.com/accessories/mp3-players/zune/zune-adapter
or
site.com/mp3-players/accessories/ipod/ipod-cover
These URLs identify everything. Product, child categories, parent categories. The main thing, they always contain your KEYWORDS. No matter what section of your site they go to, this should contain your keyword.
The breadcrumbs should mimic the URL.
Code:
Home > MP3 Players > iPod > iPod Touch
Your title should always be the URL/breadcrumbs BACKWARDS. This allows your site to identify itself as well as product pages, especially since product page titles can end up being pretty long.
Code:
16GB Zune > Zune > MP3 Players > Your Ecommerce Store
If that gets above Google's page title cut off limit, a large section of your title will be truncated.
Code:
16GB Zune > Zune > MP3 Pla..
Even if it gets truncated, it contains the KEYWORD for this particular page.
Of course you want each page to be unique in every other possible aspect as well, including the meta description. You should also have a good sitemap. But clearly laying out your navigation, site/URL structure and other identifiers will help Google to navigate your site much better. This will increase the effectiveness of your off-site campaign, link building and whatnot thereby reducing the amount of work you will be required to do. Of course, you should always be increasing your off-site SEO. Just in case; you never know who's lurking in the SERPs.
Also, I believe a good amount of quality content is just about mandatory on the homepage. I hide some on my site, it's all there, but it's contained in a slider that doesn't slide with nearly clear buttons that don't say anything. Tons of content in those back slides.
As for optimizing for long tail product keywords, if you're planning on staying a while, I would say wait to see what those should be. Focus now on getting your homepage built up and letting Google navigate your site and get the idea that you're an ecommerce store selling MP3 players and accessories. Once that's been established with Google, knocking down long tail product keywords will happen all day long on its own.. Depending on the competitiveness of your niche and how saturated it is.