I would follow this few steps to start:

• Define your target market: Who are you writing for? Why? What could be interesting for them to read? Define a list of keywords for that specific niche market.
• Content: define which content is easier for you to get and share. If you are an expert on some topic, I would say you go that way. Try to combine it with text, audio and video.
• Buy a domain name: it will make your site look more professional
• Go through social networks: start with twitter
• Deliver yur content through many channels: Youtube, Digg, Delicious, etc.
• Comment on popular blogs, sites, forums and leave a link to your site at the bottom.

Your content needs to be of quality and useful to your visitors. Depending on your niche, they can be entertaining or informative or in some cases, both. As a blogger, I find list posts (10 Tips to..., 7 Ways to..., etc.) as the most popular type of content on my blog. A useful content isn't content that you think will be useful, but content that will benefit your site visitors. Too often I have seen bloggers write for themselves rather than their blog visitors.

The above will be the content tips which you can apply to your blog. Of course, you need to get the word out about your blog. Great content is simply useless if no one is actually reading them.

Also, since, you are serious about growing your blog into something big, do consider hosting a blog on your own hosting package and domain instead of using Wordpress.com.