Establishing tone is something you want to do very quickly. Let the reader know that it’s okay to laugh, cry, be terrified etc. The tone you establish here doesn’t necessarily need to be set in stone for the whole story, but you want to give a flavour of what’s to come. Think of the overture that you might hear at the beginning of a musical: how it gives you a little of everything to prepare you for what lays ahead.
Some great examples of tone-setting first acts:
The Hitchhiker's Guide to the Galaxy’s first act is a wonderful mix of giant spaceships, planetary destruction and jokes about pubs, bureaucrats and bad poetry.
Catcher in the Rye adopts Holden’s tone of bitter sarcasm from the off: “All morons hate it when you call them a moron.”
Stephen King’s IT weaves together blood-curdling moments of horror and the history of the town of Derry with a mixture of personal anecdote and reportage so effectively you feel like you’ve lived there.
Keep an eye out for a course on tone coming soon!