There are hundreds of idioms in the English language. One very common one is, "The grass is always greener on the other side of the fence," which means, the things other people have always look better than the things you have. I'm sure there are many websites with English idioms listed with their meanings. Try searching for "English idioms" and you will probably find more of them than you can ever use.

For a native speaker, this is quite a difficult question to answer, as idiom is just part of our language. You don't ever think 'Is the expression I am using idiom?'. Think about it from your point of view - what spanish idioms do you know? And what do they mean? Also - idioms vary a lot. They vary from country to country. They vary from age group to age group. My son uses some idiom which isquite different to what I do, and my parents use some that is different again...