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Assuming you have more than a few animals, you'll need to mark them to tell them apart. This will help you track individual milk production and illness. Tagging is a common method. Always buy disease-free animals, and keep them isolated from other animals during transportation to your farm. Quarantining new arrivals (and animals that fall sick) is recommended, especially if they do not have trustworthy, recent health records. Your local government or veterinarian can give you specific advice about diseases in your area.  Equipment shared between farms can spread disease. Try to confirm where the equipment has been used and whether the animals there were healthy.  Disease-carrying ticks are a major problem for livestock. Inspect animals for ticks regularly, and keep the shed area clear of brush. Feeding cattle and other livestock can be a complicated business. There are many different kinds of fodder and forage plants, which provide different amounts of energy, protein, roughage, and various nutrients. A veterinarian or experienced farmer can help you work with the food you have available.  Mineral licks and/or mineral supplements are an important part of the animal's diet. Moldy feed or feed stored in the same area as pesticides and other contaminants can transfer dangerous toxins to the milk.  Dairy animals have high nutrition requirements compared to animals raised for meat. Improper nutrition can lead to lower milk production or lower quality milk. Milk-producing animals typically need milking two or three times a day. Move the animal to a clean location. Wash and dry your hands and the udder before milking. If you've never milked an animal before, learn how to milk a cow or goat. You will need to breed your female animals regularly to keep them lactating as often as possible. The cycle of breeding, calving, and weaning calves has implications for the animal's nutrition needs, health, and of course milk production. Our guide on cows gives you the basics, but this will vary based on species and age. Unlike farms that raise livestock for meat, you will be calving all year round to keep milk production steady. Keeping track of where each animal is in the cycle is vital so you can stick to a plan that keeps your income as regular as possible. Whether to sell, slaughter, or keep an animal is one of the toughest questions for a dairy farmer. Culling allows you to replace a low-yield animal with a higher-quality replacement, and to increase the genetic quality of your herd. Both of these factors are important, but performing them without a plan can add massive costs for replacement animals. Take this into account in your business plan, and include the cost/profit of producing each male and female calf as well.

Summary:
Mark each individual animal. Control the spread of disease. Give the animals proper nutrition. Milk the animal frequently. Understand the breeding cycle. Plan for changes in your herd.