I heard it somewhere that own designed or premium themes of wordpress makes your site more optimized for adsense then free themes. Is that true? If yes then what all have to be learned for making my own wordpress theme?
I don't think that's true... If you are looking for SEO Optimized themes then there are many open source themes that are way better optimized and go thorough rigorous testing than many Premium themes Go check out Wordpress.org's Theme Directory, you will find tons of quality themes in them. Yes, Design wise premium themes might win but as far as optimization of code is concerned... many free themes will beat them hands down...