Congratulations on your new urban planter boxes! Starting your first garden is exciting, and the absolute easiest and most rewarding place to begin is with herbs and leafy greens. They grow quickly, don't require much space, and give you fast results.
For herbs, start with basil, mint, or chives. You can buy small plants (called "starts" or "seedlings") from a garden center, which is much easier than growing from seed. Plant them in good potting soil, place your planter box where it gets at least 4-6 hours of sunlight, and water when the top inch of soil feels dry. You'll be able to snip fresh leaves for your meals in just a few weeks.
For greens, try lettuce or spinach. You can sow seeds directly into the soil. Keep the soil moist, and you'll see sprouts in about a week. These "cut-and-come-again" varieties allow you to harvest outer leaves while the plant continues to grow.
Remember, the key for a beginner is to keep it simple. Use quality potting mix, ensure your boxes have drainage holes, and don't overwater. Start with these easy plants, enjoy the process of watching them grow, and you'll gain the confidence to try more later. Happy gardening