There are several tests that you can choose from and if you are reading this, you most probably already know what you need.
Before you take the test, you should study what type of exercises there are, what kind of skills you need in order to get the best score and it goes without saying that you need to know a wide variety of vocabulary, grammar structures and have to practice listening, writing and speaking.
Take a look here at a comparison of the different types of exams that you can take.
The Cambridge tests cost from R$ 500 to R$ 600 and can be taken at Cultura Inglesa language centers. They are of the pass-or-fail variety and, if I were to simplify everything, they focus on grammar structures and vocabulary. More info here.
There are two versions of the IELTS: the Academic Version and the General Training Version. You should know which one you need to take as they are a bit different - more specifically the Reading and Writing sections differ.
The test costs R$ 440 and can be taken in Sao Paolo. It is scored in bands so it's not a pass-or-fail test.
More info about IELTS here or here.
The TOEFL test can be taken in Campo Grande. I believe that CNA offers it and it costs about US$ 210.
It is scored in points (click on the comparison link above to see how it compares to other tests) and nowadays you can only take the iBT (internet Based Test). Grammar and vocabulary are important in order to get a good score but its main focus is on skills needed at university: understanding different topics, being able to take notes and listen at the same time, slang and spoken English expressions, etc.
More info here.
You can try a free online sample test from the "Useful links" section to see what it's all about.
