I would go to say the biggest contributer to your you ranking in search engines is the amount of websites that link to yours. However the websites linking to you must be relevant and some what reputable themselves to offer any type of noticeable benefit. If you have a creditable website related to your niche with a one way link to your website that shows Search Engines that you have something that is half decent, so the more income links the better.
If your websites are new and you don't have any links point to your website then i would say start with article writing and submit your website to a few directories.
1) Writing Articles
My favorite Article submission website is ezinearticles.com, they do give you help on writing your articles but try to keep them on topic to the niche of your website and make them actually helpful to others in some way. You can find many other Article directory websites by searching Google.
2) Directory Submission
Submitting your Website to a Website Directory can help you in 2 main ways, you get free traffic from the users that find you through that directory and you also get a free incoming link. The link isn't exactly a strong one but it gets you started. The two main Website Directories you want to try and get into are
www.dmoz.org and dir.yahoo.com, start with those 2 and go from there. You don't just get added however, your site is review by an actual person and they determine whether your website is worthy of being added. So don't submit your website until you think it is ready, poor quality websites don't usually get in.
That's just a start for new websites, there is much more you can do.