Which Kenko extension tubes work best for macro with these lenses?

I want to get into macro photography!
I got TWO Nikkor lenses.those are,
Nikkor 18mm -105 mm - F/3.5-5.6 VR
Nikkor 55-300mm F4.5-5.6 G ED VR
I'm using D90 and i'm a beginner.
Can you please guide me, which Kenko extension tubes work best for macro with these lenses?
I like to shoot flora and butterflies. What sizes of extension tube should be used?
Which lenses are the best for me?

Appreciate all replies.

Just make sure that the extension tubes have electronic connectors so that the CPU information from the lens can be communicated to the camera.

From the Kenko website

"Kenko's Auto Focus extension tubes are designed with all the circuitry and mechanical coupling to maintain auto focus".

I suggest you buy the auto extension set; 12 mm, 20 mm, and 36 mm