You need online traffic to get in front of your audience and for that you should use free traffic as: Article marketing, Press release, Blogs, forums, email marketing, social network marketing, social media, traffic exchanges, joint ventures, video marketing, affiliate marketing, links exchange, Podcast.
I think you need to combine relevant content, social media, as well as traditional PR and a programmatic fashion to communicate. We have found that companies that rise to leadership positions are consistent at communicating value to their markets through all channels.
The first step is determining your target market, where they hang out, what their pain points are, what will they be searching for, why should they choose your service or product above your competitor.
Do keyword research to determine which keywords relate to your product or service.
Start with a blog creating fresh content related to your product or service. Write several blog posts to start then write new posts at least once a week, twice would be better.
Then promote the blog in social media. You need to be on all of them: Facebook, Twitter, LinkedIn, Google+, YouTube & Pinterest. Depending on who your target market, focus your time on the one where your target spends the most time.
Build followers by following potential customers, sharing related content, and building relationships.
I can help you with the set up, branding and training your staff on the strategies for each network.
First we should identify the budget. Low might mean very different things for different start-ups. It would help to know what kind of a startup it is also because depending on who your core audience is we might have to go in certain directions that best fit the personality of your end user. To throw out "social media" and "blog writing" is already a given. It really doesn't matter what your budget is, you have to try and figure out how to incorporate most marketing channels unless you are sitting on Facebook or Pinterest part 2.
Content marketing is important but if you are in a saturated space you might have to curate 3 times as much content to make an impact. It also depends on what kind of traction your startup has received during development. Maybe there is already chatter built up and once you launch you will see a quick burst of curious eyes you can quickly build off to gain even more traction.
Lots of variable to discuss before anyone can answer this questions properly.
Put out content. I built an audience of a few thousand for my startup that I launched yesterday http://inform.ly. Here is a post I wrote on all the content marketing tactics I used. Hope it helps.
You have some excellent answers already, but something you may want to try in addition is to join your local Chamber of Commerce. I don't know where you're located, but the Chamber I'm with provides excellent opportunities for exposure and offers various methods of driving traffic to your site.
Do people want to win an iPad, or an iPhone 5? Have a competition (with a stupidly simple question to which you can find the answer on your website), and then give people more chances to win the more people they tell about your company. -> Viral Marketing....
There are any number of ways of doing this, only your imagination is the limit :)
Social & PR sound great. Unsure the niche of the site, though I'd defiantly hit Forums where your users would be. You can just go to Google and type in Forum: (niche your in) and you'll see all Forums related. I'd participate and post your site if it can help the group. Thats just one thought.
All good answers - I would also look at PR.com and get some professional press releases written and space out the delivery to once a week or twice a month depending on your news level. Pay the few bucks extra it cost to include your URL in the release, because they blast these releases all over the place, and bloggers especially like to pick them up and use them for content.You will get a quick bounce within hours of the release.
If your site is something that CAN go viral, this is a good way to prime the pump.