Read Dharmesh Shah's book "Inbound Marketing" ; it is the most comprehensive and up to date overview of modern SEO and social networking marketing that I know of.
After reading it you may find that there is still quite a gap between knowing what to do, and actually getting it done. That's a bit unavoidable, as the area is technical, and also quite human time consuming (i.e. writing good content takes time). But after reading the book, you should be in a position to make some good choices, and identify key areas for your company to focus on.
Bob Walsh's book is also very good, and I would recommend it -- but if your question is specifically about marketing on the web, then Dharmesh's book is focused exclusively on this topic, and goes deeper.
For any new start-up there are many challenges, including budget. I just wanted to recommend that you take a look at the Microsoft SMB Center. It doesn't require registration, and there is a lot of great information, especially under the Learning Center and Start-Up site.
I hope that helps, and best of luck!
Allow me to illustrate so I best can explain how I market my brand with the least capital expenditure.
I am a social media marketing professional. My target audience are business startups. The solution that I offer is how to market your brand (product or service) using social media. Having defined that, I go to different online communities/forum, such as this and address issues pertaining to my niche.
I have a blog/website that I update at least once a week. This blog's RSS feed is connected to facebook, which in turn is connected to twitter. This way, whenever my blog is updated, they are automatically shared with my facebook page and twitter. I also submit articles to social bookmarking sites: digg, reddit and stumbleupon.
When a startup starts looking for help with social media communications, I have a number of linkbaits from a number of social media sites that will lead them back to my sales funnel. In that sales funnel (my blog site) potential clients will find everything they need to know about me, and more importantly, how they can best contact me.
For the last 15 months, the only expenses coming out of my pocket are to pay for my broadband connection and the site's domain name.
I have read a lot of the e-marketing books and I can tell you that what you will find is a lot of good ideas and very little rigorous analysis of whether they work. Based on my experience, here is what I can tell you about on-line marketing:
I have never had much luck selling products with web advertising. That is not to say it doesn't work, but when you are selling expensive, long sales cycle OEM tech products, it just doesn't work.
But I have had a lot of luck selling services using web marketing. A lot of luck, in fact.
Hope this helps.
I have had good results with a blog. But don't expect it to be done in a week. It can take years to grow. And you need to post regularly (at least once a week).
I think it depends on your personality: are you willing to commit to working a few hours on it each week for at least a year?
It depend of you product (you do not tell us what is it), but some viral SEO can help. I am thinking for my product a way to integrate with other services and web sites. In this way a lot of people will share the information about my site and probably link to it.