This is intended as a very brief introduction with the basics only! Thanks to @Richard who also contributed 
Maybe you want to reverse type 2 diabetes, or maybe you want to prevent disease. Maybe you want to improve athletic performance, or you are simply curious about the health benefits of a ketogenic diet. Regardless of why you are starting this journey, here are the basics!
The Basics
Eating under 20 grams of carbohydrate (ie: sugar or starch) guarantees your body will eventually enter a state of ketosis. Our bodies become good at burning fat for energy, because we are not giving them carbohydrates. Typically we disregard counting calories, as the primary mechanism of weight loss is hormonal equilibrium.
Do Eat: Meat, fish, and other protein, full-fat low carb dairy (cheeses, heavy whipping cream), eggs, and high-quality fats (lard, tallow, animal fats, olive oil, butter, and ghee). Add vegetables to meet macros. Enjoy the high fat meat you avoided for so long - it’s good for ya!
Don’t eat: Sugar and starchy foods. These include bread, potato, rice, beans, corn, and most grains. The main issue with starchy foods and sugar is that only a very small amount fits in the 20g window. Most prefer to eat low carb vegetables instead.
Macros
You may see people talk about macros on a ketogenic diet. Macros is short for macronutrient, which are the nutrients we get our energy from: fat, protein, and carbohydrates. Most food contains a ratio of these three sources of energy. For instance, most meats are predominantly protein and fat, and most plant based foods are a combination of protein and carbohydrate. Dairy foods are usually a combination of all three.
Usually, people on ketogenic diets follow a certain percentage of fat-protein-carbohydrate to maintain ketosis. What the heck does that mean, you may ask? Depending on one’s particular needs these may change. Carbohydrate is the only macronutrient not necessary for life. Protein and fat are vital for us to eat. Our protein needs differ based on physiology, gender, and degrees of activity. After protein needs are met and carbs are kept to a minimum, the rest of the energy typically will come from fat.
Examples of macro calculators:
Keto Diet Buddy
http://keto-calculator.ankerl.com/
Hidden Carbs
When starting keto, often we aren’t used to hidden carbs in things. Make sure to read all labels! Here are some places they lurk:
Spices
Sauces (Chinese, gravies)
Dairy products
Shredded cheese (cellulose as filler)
Sausages
Processed meats
Deli tuna and chicken salad
Condiments (ketchup and BBQ sauces especially)
Some salad dressings
Breaded fried foods
Cashews
Coleslaw
Low-fat products (sugar is often used as a flavor replacement)
Milk
Chewable vitamins
Cream soups