Hello there @silviam !
It doesn't matter how long you've been gardening, we all learn here!
Seeing as it's an elevated box, perhaps drilling some drainage holes in the bottom might be an idea. Otherwise using well-draining soil and water management helps great as well.
I think Mark did a vid on this that’s on YouTube. From memory he used Birdie’s raised beds and initially following their instructions used crushed granite or similar for drainage in the bottom, but at end of vid said doesn’t matter too much what you use as long as it’s not clay or similar.
Are you worried about drainage in the box, or out of the box once it hits the surface layer of the ground? If the latter Mandy’s suggestion sounds pretty good. If not, Mark’s advice would stand I reckon. What have you used to fill the box?