Run your site in OpenSiteExplorer.com and also run your top 5 competitors. To find your top 5 competitors, do a search in Google for your main keyword.. the top 5 sites are your top 5 competitors. Be sure you are logged out of your google account when you do the search.
Create an excel file that maps the domain authority, page authority, total links, and links from unique domains. If your numbers are less than any of your competitors, you have more work to do.
If you lack in domain or page authority, you need more high quality links.
If you lack in number of links from different domains... you need more links from many more sites.
If you lack in total number of links... you just need a whole lot more links.
Once your numbers outrank your competitors in terms of authority and links, if you still don't outrank them, then you need to look at on-site stuff.
This is a ROUGH overview and an easy way for a beginner to start work asap. What we do is a little more complex but we can save that for another day!
Nicole