In Minecraft, you can get Seeds from destroying Grass (not Grass dirt blocks). These seeds can be planted on tilled dirt. You can till dirt using a Hoe. Seeds need to be placed less than 8 blocks away from water and need light to grow. The Seeds you get from grass grow to become Wheat, useful for Bread and Cake (as well as helping to breed Sheep and Cows, and good for luring them into a Pen) and you can get other types of seeds in different ways. You can go to Plants-Minecraft Wiki to find out about the different seeds, and you can visit Minecraft Farming-Minecraftopia to find out how to care for your plants.