Quality keyword-targeted content is the basis of your SEO. However, I think the trickiest part nowadays is the off-page link building.
If you have the will and the know-how, you can easily produce killer content by being VERY informed or creating something truly original (gather your own data through surveys, studies, etc.)
With link-building, your best bet would be to guest-post in relevant niches. You have to do the same thing (provide quality content), except it has to be tailored to the audience of the site you guest-post to.
Also, be VERY careful on how you pitch sites for guest posts because most quality blogs (especially the ones that specify that they do accept guest posts) get bombarded with hundreds of requests. Try and make yours stand out from the rest.
And, last but not least, play by Google's rules!