Theory:

Idioms can be classified into several categories, based on the comparison used in the idiom, like animal idioms, colours idioms, sports idioms etc.
Example:
1. Animal idioms:
snail.jpg
At a snail's pace
 
fish.jpg
Fish out of the water
2. Sports idioms:
shot.jpg
Give it your best shot
 
ball in your court.jpg
The ball is in your court
3. Food idioms:
cake piece.jpg
A piece of cake
 
couch-potatoes.jpg
Couch potatoes
Food idioms - based on food items:
  1. A hard nut to crack - A difficult problem to solve.
  2. A piece of cake - Very easy thing to do.
  3. Couch potatoes - Lazy person who refuses even to move.
  4. Bread and butter - Basic necessities.
  5. In a soup/pickle - In a difficult situation.
  6. Like two peas in a pod - Always together.
  7. Not my cup of tea - Not my usual type.
  8. To butter up someone - Flatter someone.
  9. To spill the beans - Reveal a secret.
  10. You can't have your cake and eat it too - You can't have something in two ways, choose one.
  
Sports idioms - based on sports terms:
  1. The ball is in your court - The decision is yours.
  2. Give it your best shot - Make the best attempt.
  3. Front runner - The person most likely to win.
  4. Out of someone’s league - Not at a level where one is as good as someone else.
  5. Hat trick - Being victorious three times in a row.
  
Animal idioms - based on animals:
  1. Let the cat out of the bag - Reveal a secret.
  2. Lion's share - The biggest share.
  3. Wild goose chase - On a pursuit of something not about to be fruitful.
  4. Fish out of water - In an uncomfortable place.
  5. Kill two birds with one stone - Get two things done at the same time.
  
Colours idioms - based on colours:
  1. Gray area - Something which does not have clear specified rules.
  2. Green with envy - Extremely jealous.
  3. Once in a blue moon - Very rare occasion.
  4. White lies - Harmless, innocent lies.
  5. Black sheep - Odd person or bad person in the group.
Important!
Study more such idioms here.