Hi Jean i just took the exam this april, you go to this page and there is an option in English:
basically i can tell you that it does not matter were are you from, if you are living abroad there are places to take the exam, normally you can take the test in the Turkish embassy of the country you are living.
Or if you can you can come to take the exam wich take place in Ankara, this year the exam center was Gazi University.
The examen is once a year between the first 10 days of April. You have to apply normally in January in the OSYM centers or in the embassy of Turkey. You need to have completed your high school and make your high school Diploma to be validated by the Turkish goverment (you can ask how to do that in the Turkish embassy if you are in Turkey you can validate it in the Education Council in Ankara, the process take aproximatly from 1 to 3 months depending upon the country you are applying).. İn order to Validate this diploma you have to legalize or apostillate it (regarding the country you studied your high school and for doing so you have to ask in the education council of your country) if you apply in the embassy of turkey in your country is possible that they do not ask you to be translated but if you are coming to ankara you have to translate it to Turkish and make this translation legal before give it to the Education council of Turkey.
Afther applying for the exam, they will send you by post mail a identification card with certain number that will serve you to take the test and know the results from internet. They will tell you also the date of the exam and the exact place you can take the exam.
What to study in the exam? well the exam consist of two parts the first part is conformed by 80 question from which 40 are mathematics and 40 IQ questions. The second part consist of 80 Turkish questions. So you have to study almost all the topics of Matematics like numbers, rational numbers,ratio and proportion, factorization, exponentials, first order equations, roots, inequalities and absolute value, sets, polynomials, functions, second order equations, trigonometry, logarithms, complex numbers, induction, limits, the derivate, integral, matrix and determinants and Geometry.(from matematics Geometry take 5 questions).
You do not have to know deepiy matematics because the exam is not too dificult but you have to be able to know every topic well and know a short and fast way to solve the matematics and IQ questions. One of the most important things is to be able to solve as fast as you can the two parts of the exam (it takes 3 hours: 1.30 for Math and İQ questions and 1.30 for Turkish part). All the instructions are in English and Turkish. And one important thing is that you just answer the questions that you are sure you know the answer and never try to guess other wise for every 4 incorrect answers they will eat up one correct answer and it will decrease your points.
The minimal points that you can get is 40 and it will depend upon the university you want to study, not all universities in Turkey are accepting foreigner students and the best Universities are asking for high points for example to be able to study in Bosphorus University you have to get at least 75 points (the maximun is 80) and A value in the Turkish exam. Every university that accept foreigners students have a quote by faculty and from every country of the world 5 students with compite among themself in every university of Turkey.. Most foreigners student want to study in the best ones so the exam is very competitive ...
Oh i forgot something very important: As you are living in USA you can apply for another option which would better for you: TAKE THE GRE EXAM! if you get high points in that test you will be able to apply to any university in Turkey that accept foreigner students, this is better option for you...
İn YÖS exam is better to come to turkey at least 6 months before and get in to privates curses which are specialized in prepare people that want to take the exam, like this one:
Again ask in the turkis embassy in USA that you will take GRE but you do not know what would happen with the Turkish part...obviously you would have to take Turkish courses other wise afther being accepted in a Turkish university you will have to take one year of Turkish intensive courses to be able to start in the program you have chosen...
İn my case i know my puntuation right now iam waiting to know if i would be able to get in the carreer i want to be in or if i will have to take YÖS exam again...if it is the case i would better go for GRE.
Good look to you!