I want to use my jquery + js knowledge and add it in my joomla site.
I was thinking to use JUMI and create js components? Is it right?
You can put your code in the header of the index.php file of your template.
If you want to use Jumi, you can watch the video tutorial at here: http://joomtut.com/joomla-extensions/52-jumi-the-set-of-native-custom-code-extensions-for-joomla.html