There are 2 categories of seeds 1) world generation seeds 2) food source seeds. Of the food source seeds there are 2 types, plain seeds and root seeds. There are 3 types of plain seeds A) wheat: green in color, B) melon: black in color and C) pumpkin: white in color. There are 2 types of root seeds A) carrots, B) potatoes. All seeds can only be planted in tilled soil by using a hoe and the soil must have a water source otherwise the seeds will uproot.
Wheat seeds can be obtained by destroying blocks of grass, in game chests, or trade. Carrots and potatoes can be obtained by harvesting, bonus chests, trade, and world generated chests. Pumpkin seeds from destroying a pumpkin, trade, or inside of world generated chests. However, melon seeds can only be obtained by trade or in game chests.