You don't need to freeze the substrate. And cleaning hides, water dishes, etc before introducing your snake to everything is definitely recommended and MUCH easier to do now before you have the little guy/girl.

And as mentioned, 20 long is big for a hatchling so you'll want to create as much clutter in the tank as possible so they feel they always have a secure place to hide. Also, it's recommended to clean your snake's habitat once every month or so, so get used to doing this. It's a big part of keeping your new snake happy & healthy.

Would also be good to know what you are using for substrate & heat as well as the general environment you live in (warmer area, colder area)...