If you had to guess what the most common SEO mistake would be, what would you guess? Building a site of all images or in Flash? Not making a title tag that says home versus your core keyword phrase? Spamming Google? Nope.
Matt Cutts, Google's head of search spam, said it is not having a site at all.
In a fun video, where Matt goes invisible for parts of it, he says that is the number one mistake. If you do not have a web site, then Google does not know you exist. You are invisible to Google. So number one priority, get a web site.
Here is the video: