You are getting there, a good image.
A few good points above about focusing.
One approach with a close up like this would be to select manual focus and live view and use the live view magnify feature to zoom in on the eye and tweak focus manually.
I would stay with a smallish aperture to ensure full depth of field (focus back to front on subject) which is important with this type of image.
You used a pretty good lens. I personally would use your lens at F20 without too much concern about diffraction if I needed a small aperture. It is more of a problem with cheap kit lens.
To ensure the sharpest image I would focus on eye and take a series of exposures at say f22, f18, f16, f11 and f8 with camera set on apreture priority setting and examine later for full depth of field. I'm assuming the creature will stay still.
You will have a depth of view preview button on your camera, this stops the lens down to the set aperture to show what's in focus. You may need to use a brighter light whilst you do this to be able to see details. There are depth of field calculators (google it) which give you an idea. Difficult to tell looking at your images as we have no idea of size and distance.
Like Paul says a dedicated macro (when you buy one!) such as the nikon 105mm f2.8 I use will give very sharp images at f22 and smaller
When viewing your image the main point that occurs to me is the composition. The geko? creature slopes down whilst the reflection is horizontal. I feel it would be better to see both bodies coming in from the left with background above. This may not be possible due to the mirror effect.