MyBB Community Forums

You'll need a plugin to do that. As far as I know, there's nothing like that, but you can place a request in the Requests forum:

http://community.mybb.com/forum-65.html

thanks, posted in requests

(2011-06-06, 07:55 PM)Yaldaram Wrote: [ -> ]

(2011-06-06, 07:50 PM)Aohor Wrote: [ -> ]I ll also show ads in my all threads 1st post.so is it more profitable for me???

For this, you might find this plugin useful: http://mods.mybb.com/view/my-advertisements

Where to put this code please explain

@Aahor: It would be better if you ask for MyBB related support only, and not about adsense earning tips. For your question about best way to add adsense, Myadvertisements plugin, suggested already, is the best choice.
For any questions about other things like Adsense earning methods, please post a separate thread in General discussions section.

@ SonicBusa and badubanis: We can help you better, if you start a separate support thread, instead of hijacking another.
Answering badubanis's question, its not a code. Its a plugin. Find the installation manual in the download file.