Check out this module for adsense, or any cut and paste code for that matter....
extensions.joomla.org/component/option,c...ink_id,25/Itemid,35/
Or to disable your wysiwyg editor like you asked do this:
1) Log in to your site as admin in the backend.
2) Navigate to site-> global configuration from the top menu items
3) On the first tab shown "site" there is a pulldown next to an item named "Default WYSIWYG Editor:"
4) Choose "No WYSIWYG Editor "
5)Click the save button at the top
All done! you just disabled your wysiwyg editor!
Next thing to do if you want to create your own module is this:
1) From the top menu again go to modules-> site modules and click.
2) Now click the icon that says "new"
3) Name your new module something like "cool new custom module" or whatever in the title field and set it to not show title.
3) Deselect all the options about caching, feeds and mambot etc.
4) Paste your adsense code or whatever custom code, javascript or whatever in the blank field labeled "custom output"
5) Click "SAVE" Publish it and locate it where you want in one of your templates module positions.
Hope this helped!