Since I'm not from an English-speaking country, schools require an additional test for that language. There are two main tests which may be used for that purpose, the TOEFL and the IELTS. I used my TOEFL score for most of my applications.
The TOEFL is a test which takes about 4 hours and evaluates your English proficiency in 4 sections: Reading, Listening, Speaking, and Writing. You get a score which range from 0 to 30 for each section, so you can get a total score between 0 and 120 when adding all four sections.
The minimum score you need to get depends on the school. There are school with minimum total score requirements. There are schools with minimum requirements for specific sections.
In order to have a TOEFL report which can probably be used for any school, your total score should be at least 105, with at least 25 in each section.
It seems to me that, once you meet the school's minimum requirements for the TOEFL, they don't worry much about it anymore. So, if a schools only requires a minimum score of 100, it will not make much of a difference if you get 110 or 115, for example. So, contrary to the GMAT, trying to get a perfect score in the TOEFL will not really improve the strength of your application that much.
Also contrary to the GMAT, I did not spend so much time preparing for the TOEFL. But that's because I use English everyday in my life. Almost every book I read is in English, and it has been that way for 25 years. I had a job where I had to write in English every day, for hours and hours. When I watch a movie, I usually watch it in English. I got a perfect score of 30 in listening, 29 in reading, and 28 in writing. But I got a lower score of 23 in speaking, since I don't really have much of a chance to talk in English often enough. And I must say that the English section of the TOEFL is awful. You don't speak with a person, you just answer stupid questions into a mike in a way that does not feel natural at all.
If you are already good in English, just take some time to learn the details about how the test is structured, what kind of questions they ask. Please notice that, even if there are 4 distinct sections, one section may require skill from another section. The writing section asks you to write about something you listened to, for example. So, if you aren't good at listening, you may not be able to really understand hat you should write about.