Even though you have an online store, I think many of the tips within this article can help you. Its a long read, but you will start to see how these things are interconnected.
For the beginning: Start simple. Don't get overwhelmed. Don't think of the whole pie, think of the pie in slices:
1. On page optimization and Titles: Page titles, meta, on-page content. In any basic optimization, this is where you start. Google and other search engine providers need a clear idea of what your site and pages are about. Make it easy for search engines to see what your site is about.
2. Keep your message clear and consistant. If you have a page about mens underwear, make sure that page talks about mens underwear. If you have additional products or product variations, create pages that are relevant to that products qualities and differences. Additional pages = additional ranking opportunities.
3. Know your market: Look at competitor sites and Google's keyword planner to use the keyword phrases that matter. If a certain keyword has over 10000 searches per month, and another has 1000, go for the bigger phrase if it makes sense to your product and your website. DONT GUESS. Guessing what people are searching for can be the biggest waste of your optimization efforts and time, and reversing bad keyword choices will take additional time and effort to correct.
4. Be technically correct. On your site, look at page load speed, image optimization, and anything that contributes to your site being accessable quickly from a wide variety of devices (Desktop, mobile, tablet). Make it easy on your users to absorb what you display. Quicksprout has a simple tool that can help you see potential load time issues.
5. Let search engines know you exist. When you are finished creating your pages and site, create sitemaps with all your page urls and submit them to Webmaster tools (both Bing and Google). Use Google's fetch feature to make them aware of changes, and speed up the indexing of your site. Do not wait for them to find the updates on their own.
6. Check for errors. Look for duplicate pages, or pages that display across mutiple URLs. If you have pages that behave this way, redirect them so that search engines know only one page exists for any given item or page.
7. Track your progress: Use MOZ or another keyword tracking tool to see the changes in your ranking. After all the hrad work, nothing satisfies like seeing your ranking increase and knowing you are on the right path. Use Google analytics to see where people stay on your site, and where they leave in the bounce rate and exit rate report. Use that data to further increase the usability of your site and keep people engaged and interested.
If you already know of competitors that do well in your segment, I would do a competitive SEO analysis.