In the menu,
Tools > Preferences you have
Automatic substitution (make sure it's checked). There you can enter words you want to have automatically changed.
This would mainly be used for common misspellings, as it requires you to enter all the corrections yourself. I don't think there's a auto-correct function as on your phone, which automatically chooses the spelling it believes is correct.
If you misspell a word, you can right-click it and choose "Always correct to [correction]", helps speeding up the process somewhat.
As of now it does not seem to work with multiple words, so adding "helo world" doesn't work. It will only recognize single words. So changing "helo" to "hello" will work.
Note that it's case insensitive. The replacement will have the same case as the original word ("Helo" > "Hello", "HELO" > "HELLO", etc)
You can read more at the support article. There you can read about "Spelling suggestions" and "Autocorrection" as different things (you'd want the combination).