There are many ways to do advertising online. Let me put some example for you to see,

1. Search Engine submittion.
2. Social Network (Hi, Facebook, Youtube).
3. Pay per click advertising.
4. Directories submittion.
5. Articles Submittion.
6. E-mail Marketing.

     What are they? I would like to explain each of the samples I picked up. The first one will be Search Engine Submittion which will allow major search engines to detect your website.
These search engine are very useful to bring people into your site. You can go for free submittion from Goole Submit. If you type down "google submit" in a google search box.

      2. is a social network advertising. You can always attach your links to your hi5 for facebook account and start to add friends and create your own social network. These traffics
will be re-direct to your site for free!.
      3. Pay per click advertising (PPC) is very common and easy to use. PPC can become handy but high cost to advertise. You can set-up your own advertising budget each month with Google adwords.
      4. Directories submittion is where you will find lots of website with in a directory website. This thing works like Yellow Pages.
      5. Articles Submittion is very powerful tools to use. You can publish your own article online through blogging and submittion websites in which will allow web master to find your article and they will
select them for their work and put reference back to your site. This is called backlink creating
      6. E-mail marketing: if you already have e-mail of your clients then you can go for sending e-mails to call them in.

hope this is a useful information to you. thank you !