@Amruth: do you mean the index page, if yes you can go on the index page, click customize and then just click on the little x in the corner of the widget you want to remove. Then drag & drop custom widgets ware you like to place ads and just edit the custom widget and past in the ads html code :)

You could just customise the main page and remove the users and newsfeed widget and place custom html boxes there with your advertisement code.

when logged in as an admin you see a customise this page button at the top of the main page.
To remove a widget, while in customise mode, click on the X on the top right corner of the box and it will be removed.