You can breed crops in Black Desert PL  to get more seeds so once you have acquired a single seed, you can plant it and then get more. So technically all you need is a single seed to get started.

Seeds can be gathered from Bushes/Thicket using barehands or gathering hoe you can purchase from material vendors or crafted (requires Tool workshop). This is a random and slow method (depending on your gathering level, most weed at low levels) but you can find random seeds for free this way while leveling up your  .