in nonfiction proposals (especially self-help books) where the audience usually didn't buy a book from a name they hadn't heard before, but now more and more agents are expecting debut authors to be working behind the scenes on building up their platform. Now, in this case, platform almost always translates to web presence. This is looking at a very narrow part of the actual meaning of the word platform, but it'll work for today. Plus, it's usually all agents and publishers expect from a non-published author.
From the agent's point of view (according to a recent interview I heard), you can get away with having a simple, free website with your name, email adress, bio, and a little bit about what you write, but you should still have something with your name on it floating out in cyber space. You can do this very easily through free platforms like Blogger, but you have to be careful about this because people expect a blog to be active. A "dead" blog is usually seen as a bad sign. Also, consider buying the domain name for your name (or pen name) before someone else does. You can set up a Blogger or Wordpress blog to redirect to a custom domain name (which is what I've done on my site). Even if you don't know how to set up the website, the cost is minimal and you'll have it down the road when you need it. But if you don't want to mess around with websites and domain names, try to at least set yourself up on popular social media sites.
I mentioned Robert Brewer's blog My Name Is Not Bob the other day and how much fantastic advice he had on that blog for writers. One of the topics he speaks about is self-promotion and online web presence, especially through social media. A relatively recent post called The Ultimate Guide To Social Media For Writers is a pretty detailed look at, well, social media for writers.
A caveat. While you should definitely make time to set up profiles on multiple sites and visit them all at least once a week, don't let your entire day get eaten by the interweb. Your main job is to write, so that should still be your focus. No matter how popular you are online, you can't get published if you don't have anything to publish!
Domain name sites (a few of them, anyway):
Instructions on setting up your domain name