how to resize thumbnail image

First of all: add_image_size() is to ADD a new image size into your WordPress installation.

Now, as you said:

I just want to resize my Image

You need to enter into your wp-admin panel. Then Settings » Media, and change the thumbnail size there.

Just change the thumbnail size there. Now use the plugin: Regenerate Thumbnails by Viper007Bond to make your new image sizes.

So, after then, if you use:

the_post_thumbnail( 'thumbnail' );

it will echo the new thumbnail image (120 x 120).

WARNING: Changing image sizes and regenerating new image sizes, doesn’t delete previously created sizes, just add new sized images into the server space. So the process is somewhat space-killing theory, if not smartly handled.