If you don't have the owner's manual, you can often find a downloadable PDF version online by searching "[Year] Toyota Sienna owner's manual" on a search engine like Google.
The fuse box is typically located under the hood or inside the vehicle's passenger compartment (often under the dashboard). Look carefully at the fuse box diagrams provided in the manual – the horn fuse might be listed under a more general category, like "accessories" or something similar.
