You can build a blog pretty easily with WordPress or Blogger. All of the tools are set up for you, and it's free. GoDaddy is the host for my website. They have tiered fees based on what you want from them--everything from registering a domain name to using their software to design your site (I give their software a B-/C+.)
As to using a blog and website to promote your book or create a platform, think first about your expertise and what you want to share. You've written a nonfiction book? Then use your site to post small articles and/or links related to the subject matter of your book. You can generate followers for your blog or site with linkbacks (when someone else links back to your site), with relevant links on your site (that draw people to the site), and by participating in various forums that are about your subject matter. Don't use the forums, though, just to promote your blog/website, or you'll be viewed as a spammer. But it's perfectly okay to post your thoughts, opinions, or knowledge on a forum and place the URL to your site in your signature. If you're helpful, people will take a look at the site.
Also, be sure to mention your upcoming book on your site. Whet people's appetite a little, letting them know you have a larger project out there that may be of interest. Blogs always do better when you provide frequent updates (at least once a week). If you run out of ideas, look for guest bloggers or people to interview.
Remember, it takes time to build an audience. My website has been up for several years and receives a lot of hits (GoDaddy's counter erases every time I top 16K; I have no idea why!) My blog is newer and is just starting to pick up followers.
Just some ideas on how you can get started....Hope that helps.
JeanneG
http://www.jeannelyetgassman.comhttp://jeannelyetgassman.blogspot.com(See how cleverly I inserted my blog and website into my signature??

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