Your new planter boxes will need two things before you begin: a coating inside (to keep them from rotting) and drain holes. If the planters are glued, add some screws. Rocks don't make a good substitute for drainage. If the boxes come without drain holes, drill some.
The biggest need for any plant is good light and proper water and nutrients. Plant things together that have similar requirements. For instance, cucumbers require a lot of fertilizer but basil doesn't require any.
Check the bag the potting soil comes in. A lot of potting soils already have fertilizer in them - adding more would be harmful. Adding a little rotted manure to the cucumbers would be beneficial but, only to them. Don't buy moisture control potting soil.