Dirt. Six inches to a foot deep.
Basking lights. Spotlights that get the basking spot to 135F+.
And cover 75% of the lid. Keeps humidity in.
I'm guessing you're very new to reptiles. Nothing wrong with that. I'm surprised they even sell hot rocks anymore...
There's a lot of garbage care sheets on the web. Do LOTS of research.
Google "varanus" and start looking. I'm not going to write you a 45 page care sheet on varanids, but I'm thinking maybe I should...
Chris