Clean the outer rind with cool water. Use a large knife to split the melon in half, then quarters, then start cutting into wedges.
Another way is to split the melon in half and use a melon baller to scoop out balls of the watermelon flesh.
You can also slice off opposite ends of a watermelon and stand it up on its flat side. Shave off the rind now that the melon is stable, then cut into cubes, plate-sized coins (makes for impressive presentation), or even rind-less wedges. Here’s a handy video showing cutting tips from the National Watermelon Promotion Board.