You should already be on the main branch: your "default" branch. You can always check with git branch.
git branch my_new_branch
This creates a new branch called my_new_branch. The thing is, I rarely use this command because usually I want to create a branch and switch to it immediately. So I use this command instead:
git switch -c my_new_branch
The switch command allows you to switch branches. Including the -c flag tells Git to create a new branch and switch to it.
When you create a new branch, it uses the current commit you are on as the branch base. For example, if you're on your main branch with 3 commits, A, B, and C, and then you run git switch -c my_new_branch, your new branch will look like this:

Run and submit the CLI tests.
The switch command was introduced in Git version 2.23.0, so you might not have it if you're using an older version of Git.