A good place to start is to check your competitors' backlinks. Analyze the sites that rank well for the keywords you are trying to rank for. You can use any backlink analyzer tool (http://tools.seobook.com/backlink-analyzer/)
. Another useful tool is Yahoo! Site Explorer (http://siteexplorer.search.yahoo.com)
Once you have a list, go after those sites and try to get links from them as well. Of course, even if you get all of those links for yourself, you will still need more. Think about your industry, your niche, and try to look for sites that might be interested in linking to you.
That being said, if your site provides little value, you can just skip this advice all together. In that case, think about how to provide value for your visitors/industry, be it tools, guides, unique products, etc.
That's why the "content is king, link is key" debate is missing the point. If you produce good content and just sit and wait for links to come in, they won't. But if you don't produce good content that people will be willing to link to, you won't succeed even if you actively try to find links. No one outside of paid directories and reciprocal link farms will be willing to deal with you.