You can put it under the 'contact us' on your website, as forum signature, or just put it on friendster profile if you want get a lot of chatmates. Quite fun and useful. Here are the simple steps:
1. Copy this code:
[a href="ymsgr:sendIM?[span style="](YOUR-YAHOO-ID)[/span]"][img src="http://opi.yahoo.com/online?u=fathah_zainuri&m=g&t=2" /][/a]
2. Paste it on a text editor (notepad)
3. First, change (YOUR-YAHOO-ID), then change [] with <> (all pairs) with your active yahoo ID so it will be a HTML code
4. Put the code anywhere in your website, blog post, profile page, or forum signature (it works at many websites, but not all)
5. You've just told the world that you're online ^_^