Idioms are linguistic conventions in which certain words go with certain other words, and English is a particularly idiomatic language. Kids usually learn idioms in the sixth grade, and then forget about them until they have to study for the GMAT and other such tests.
Idioms are often arbitrarily constructed: “talking to someone” and “talking at someone” mean two different things; a lot of people mix up “different from” and “different than” — incidentally, “different from” is the correct construction. So they’re easy to mess up. The best way to get good at idioms is to familiarize yourself with as many as you can until you internalize the correct construction.
We did some snooping around (so you wouldn’t have to) and found that possibly the most exhaustive and comprehensive collection of idioms online is at winningprep. We also encourage you to purchase Grammar Smart (or similar). The most effective way to do it is to read a ton of idioms, so get on it.