A bucket is crafted by using three irons in the crafting grid. You craft it like a glass bottle, except you use iron instead of glass. If you want to have a better overview of crafting a bucket, click on the link glass bottle and check it out. You will need a crafting table.

Buckets can be used to carry fish, lava, water, milk, or most of every liquid. To get a fish / axolotl in your bucket, right click water first to get a bucket of water. Then, find a fish and right click the fish while holding your bucket of water to hold it in your bucket.