Use a blog site as your web presence's content management system (CMS)--essentially this is where you log in and make updates to you site or write a new blog.  You can even usually purchase the domain name you want for an affordable annual fee if you want "yourcompany .com" as the site name (the cost is usually between $15 and $25 per year).  Blog sites are nice because you can choose a template that already exists if you're not ready to design something, or you can sometimes manipulate the template code, or create your own code, if you want to customize it for your own use.  You can add pages that act just like traditional web site pages, yet you have the freedom to update content and add new blog posts at will, which isn't always easy if you hire someone else to design a traditional web site for you.  There are several blog sites available to choose from...WordPress, Blogger, Weebly, and many more.

Build a social networking approach that makes sense for you and your target audience.  Use Facebook, Twitter, LinkedIn, MySpace, Digg, etc...whatever makes sense for you and only as many as you think you can reasonably manage.  Add these to your blog site.

Content is king.  For your blog site and social networking sites to all work well and work together you have to regularly create new content and participate in conversations to build relationships with those in your targeted social network...you're really building a community to create reciprocated conversation.  Meanwhile, you're positioning yourself as the go-to-person for the area of expertise you/your company focuses on.

It is important to participate in conversations.  You can create as much content as you want and publish it to the world, but you'll gain more credibility more quickly by also participating in the conversations related to your area of expertise on social networking sites and blogs.  This shows that you pay attention to whats going on in your "community" or industry.

Finally, simply by having all of the content from your blog site, and blog posts (with various pertinent links embedded in them) and all your social networking sites linked from your blog site as well as your participation in social networking conversations. search engines with crawl out and grab what content they can to push your search results higher and higher.  The typical rule of thumb is that the more content you have the higher your search results (to a large extent that's true, but this concept does have a few other variables involved...you can do some more research here).