If it is an internet business, you must first have a website with good look and content. After that you can promote your website by

- posting signature links in forums and blogs
- social bookmarking
- launch affiliate program
- build a network of fans in facebook, orkut, linkedin etc.
- try PPC, CPM advertising

I also recommend that you attend events and meetups that include the type of visitors you want on your site and just plug your site. Give them a catchy business card with your website url very visibly seen.

In my opinion, SEO should be the first step.

Yes, I agree. But SEO thing is costly. If you have an enough budget then you should first tune your website at search engine point of view.

using youtube, twitter, sitepoint market place. You can promote your online business along with its features. Some ways may not be good for it. Good luck. Hope you will choose a better way to promote your online business.

Write good informative articles and post them in high traffic article directories like EzineArticles.com, GoArticles.com, ArticleBase.com and ArticleCity.com etc.

You need to leverage both offline and online sources to promote your newly established online business. It also depends on the type of business and segments your business caters to.

For example,
Linked In - include your business post, create a business profile, join groups and include links for press release/white papers and any other documents that are of interest to the readers.
Twitter - share your thoughts about non-business news but also share links to news your Followers care about.

PPC is easiest way to get immediate traffic, though it is costly. Better have a good site with good content before you worry about traffic.

i would like to say the first thing to do is to build a blog to promote it