To fill up a bucket in minecraft, there are two ways to do it,
Go to a water source, aim with your cursor with the bucket in your hand and right click, this will cause the water to leave where it was and go into your bucket.
Using the same technique as above, aim your bucket at a cow and right click and it will fill it up with milk.
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Hold the bucket in your hand and right click the water.
Craft a bucket and Right-click on the water with the bucket.
Use a bucket
look at water,lava,a cow an press LT
Put your bucket in your hotbar and select it. Whilst holding your bucket, go up to the water and right-click on the water. Your bucket will fill up with water, which you can then pour into another location by right-clicking the spot that you wish to place the water in. A bucket can be crafted from three Iron Ingots. Another useful note, you can also collect milk in a bucket by holding a bucket and then right-clicking a cow.