I'd say watching English shows, movies, reading books, articles and finally speaking with native speakers in order to learn new idioms, understand the meaning of the idioms and when exactly to use them!

Do an online search for "most common idioms in English" and you'll find them. As native speakers, we could tell you one or two that we like, but we really can't give you a decent answer. There are thousands!!

Idioms, like slang and sayings, are personal and regional. There isn't any master list of "most common." And, unfortunately, some of the most colorful and interesting aren't really used much. Ask someone "Have you ever heard anyone actually say 'It's raining cats and dogs,'" and some people will say "Never," some will say "Certainly, all the time" and some will say "Oh, my aunt Vera used to say that." Idioms are fun to learn and useful to know if you hear them used, but be careful about using them yourself. They will probably not improve your English. When we hear even a trace of an accent we are not expecting to hear idioms, and if they are not said correctly they may be hard to understand. For example, there is an idiom, "That's not going anywhere," meaning "that's well-secured, that's firmly attached." I once had a friend who was a native Spanish speaker use the phrase--and I didn't understand it and had to ask him to repeat it. Why? Because a native speaker says it with the pattern of emphasis, "That's NOT going ANYwhere," and he was saying "That's not GOING anywhere."

Here are some that I like to use: A piece of cake = easy e.g. That exam was a piece of cake. Close shave = something almost happened e.g. That was a close shave, a car nearly hit me. It's raining cats and dogs = it's raining a lot

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