Once you've properly rinsed the zucchini, you either can leave the skin on or peel it off.
If you wish to peel it, first cut the zucchini into 2 or 3 pieces to have a better grip and peel more evenly.
If you want to slice it in rounds, be sure to keep a good grip to prevent it from slipping.
If you're making a ratatouille, you can stack up the slices and cut them into quarters.
To make cubes, first cut one side flat to create a stable base.
Hold it firmly, and slice it evenly.
Stack up the slices to cut them into sticks, then stack the sticks again and cut them into cubes.