SEO first breaks down into offsite and onsite. Offsite is simply not on your website and onsite is, yeah you guessed it, on your site. You can hire a SEO company or person to do offsite SEO for you and they never need access to your site. Offsite breaks down into these important things.
Blogging(Fresh Content) – Blogging for SEO purposes can be done onsite or on another blogging platform like Blogger or WordPress. A blog post is an article written about your website/business. Having that same blog post to different pages of your site using different highlighted text sections of the article (Anchored text links).
Backlinks – Your website gains Google search engine position or Rank with the quality and quantity of the links you have pointing to you site from other sites. Similar types of sites or sites based in similar areas can really help one another by linking to each other.
Business Directories – Sites like Yellowpages.com YP.com Yelp.com Angieslist.com are a get place to have your business listed and have your site linked in these listings.
Social Media – Every Social site is being tracked very thoroughly by Google amongst other entities. Every time some shares, hashtags, likes, follows or even mentions your site Google knows. Having people engaged and sharing content via Facebook.com Twitter.com and Instagram.com is a very good thing.