First, you gotta make a hoe, or sythe. You do this with two sticks and two wood, cobble, gold, iron, or diamond and shape them like this:
Hoe.png

Next, you should collect seeds. Seeds come in many different variaties. First, lets focus on wheat.Wheat seedsYou can get wheat seeds by breaking long grass, small trees *not saplings*, and dead bushes, like those found in desert biomes. You won't get a seed every time, but you will if you keep breaking those small plantsCarrots and PotatoesCarrots and potatoes are the only foods that can be both the seed and the crop. They both can be found naturally growing in NPC villages alongside wheat. Carrots and potatoes can also be obtained as a rare drop from killing zombies. Happy slaying! Also, the potato can be baked, unlike any other minecraft crop, and Carrots... Have a LOT of omega 3!Pumpkin seedsThese are alot harder to get. The easiest, least deadly way is to look for a naturally generated pumpkin patch, which you can find in any plains, forest, tiaga, or extreme hills biome. Although, don't be fooled. They are not at all common. If you find one, you should place a pumpkin on your head. Or just craft it into your desired seeds, like so:
Pumpkin Seeds.PNG
Yeah. Thats a pumpkin.
The much harder way will be covered under melon seedsMelon seedsThese are extremely hard to get. First a little tip: have a lot of torches, alot of food, and wepons. You need to find an abandoned mineshaft. Heres a quick video on abandoned mineshafts.







That's the main danger in abandoned mineshafts. Falling, lava, and mobs, specifically the poisonous cave spider. As of recently, all abandoned mineshafts now have bridges when there isn't a block beneath them, so the ravine part isn't a worry anymore, but it's still a good place to start looking. Once you find an abandoned mineshaft you need to look for boxes. Eventually you may find a box containing either melon, or pumpkin seeds. Then you can get outa there.
Cocoa beansYou can find them in three places. One: in a dungeon. Two: In an abandoned mineshaft. And three: in any jungle. The best way to get your share of chocolate is number three. You can find this lovely plant growing on the side of a jungle tree. It should look like this

Cocoa_Plant.gif
Stages of Cocoa
If you break it when it's small, you will get one cocoa bean, if it's big, you will get three. You can farm these special plants in a special fashion. You don't need a hoe for these. All you need is one or two pieces of jungle wood. Then right click the wood, while your holding the cocoa, and it will become a small cocoa pod. Then just wait for it to grow.
cocoa farm.png
Sugarcane and Cactus
While they aren't seeds, I still feel like I should talk about them, since they are still plants obtained by farming. A sugarcane is the only reed plant in minecraft, and cactus is the only plant that will kill you. You can find sugarcane in most biomes, excluding snow biomes and mushroom islands. It's extremely common in the desert biome along cactus, which I will get into in a moment. To plant sugarcane it must be placed like a block exactly 1 block away from at least 1 block of water, and it only grows on sand, grass, and dirt.
Sugarcane.png
Naturally Occuring Sugarcane

Cactus, like sugarcane, is indigenous to the desert biome. It can only grow on sand, it can be planted by placing it like a block onto any block of sand you desire. Don't hug cacti though, you will get pricked to death!
You can farm them by mining either the bottom block or first to bottom block on either sugarcane or cactus plants. this will drop all the blocks above the root of the plant.

Farming-Note: Do not do the following for cocoa, sugarcane, or cactus, only melon, pumpkins, wheat seeds, potatoes, and carrots-
Now you need to take your hoe and right click any block of dirt. It should become farmland, or essentially tilled soil. Now you need to take your seeds and, while you have them in your
hand, right click while looking at the farmland and you should see either a green speck, or tiny grass like blades. Now it's a matter of waiting, unless you use bone meal, which works as a fertilizer, and will grow crops instantly, grow giant mushrooms, and on grass, will make patches of flowers and small grass.
Wheat.gif
Stages of Wheat
Seed_Stem.gif
Seed Stem Stages (Pumpkins and Melons)
Carrots.gif
Stages of Carrots
Potatoes.gif
Stages of Potatoes

For the wheat, potatoes, and carrots, it's the end of the line. For the pumpkins and melons, once the stem is done, you then have to wait for a pumpkin or melon to grow. You need to have one block open on any side of the stem, otherwise you won't get melons or pumpkins. Heres an example.
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MadhatterXII's Successfull Stem Farm

Also, The best way to farm is with a bar of water for every 2 bars of farmland, as shown above. Water keeps soil hydrated and speeds up crop growth. It also helps crops be less trample-able. If you, or any other mob steps on farmland, the crop will die, and the farmland will become dirt again. Fences are ideal. Lastly, if a crop is dark green it means it's done. for wheat it means it's time to harvest, for stems, it means it's time to wait 1.5 minecraft days to harvest. For Carrots and potatoes it's actually opposite and the crop gets a little bit lighter when it's time to harvest.
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Now you know how to farm in Minecraft!