When you click the "Test" button, it runs the "Sample Tests" code. You can write your own test code there.
When you click the "Attempt" button, it runs the hidden tests which you need to pass in order to finish the Kata.

The output window will show how your code did in each test, commonly in some format like "passed" or "Expected: 1 -1, but was: 1 0".
The exact output depends on the programming language and how the tests are written, some might be easier to understand than others.
In a some case people might be able to figure out what your mistake is simply by what the test error output is, which means you don't have to show your code and mark your post as spolier.

When the "Attempt" test runs without error, the button will change to a green "Submit" button instead.

• Kata trainer reference: https://docs.codewars.com/references/kata-trainer
• Some help with solving: https://docs.codewars.com/training/troubleshooting
• Codewars Discord if you want to talk and ask some questions: https://discord.gg/7U9t33jrgG , there's also #beginners channel.

I had assumed that the main things needed would be already be imported for us to solve the Katas.

No, it's not the case. Usually you are provided with some initial stub, but you need to make sure that all the required code is there. With some exceptions, there's usually no code which is added to whatever you write and submit.

See the docs on information on how to use the site.

And this is the comment section specific for the "Highest to Lowest" Kata, I don't think there is a global messaging section.

You might want to mention which programming language you are using, as I think almost all programming languages have a Sort method, and your issue might be specifically related to which programming language you are using.
If you can tell us what the output of the test is, it might be easier to see what your issue is. If that doesn't work you can share your code, just remember to mark your comment as "spoiler".

I had assumed that the main things needed would be already be imported for us to solve the Katas.

This definitelly depends on which programming language you are using. But for this Kata, no, the needed imports/usings are not always included. I think it would be too easy if it was, and I might be slightly evil.