Well, I'm Brazilian and i'm studying a lot to apply to American universities then I will give you some tips.
Foremost, I bought an English Grammar Book four months ago and since then I've been studying English.
The name of the Grammar Book is "English Grammar in use" and i highly recommend the study using this book.
It is known that American Universities are very hard to get in, you have to do your best all the time.
You have to do a lot of tests and you need to do well in all. You probably have to do SAT/ACT and TOEFL.
In Brazil we have a test that is similar to SAT, the ENEM, therefore I've been preparing for SAT and ENEM since the fist year of school. SAT in not the hardest test in the world but you have to do the questions really fast. It is recommended that you do a couple tests to practice, I also recommend you buy a couple of books to help in the study.
There are the " Official Sat Study Guide" and others books from Kaplan that are very good to help in the practice.
https://www.amazon.com/Official-SAT-Study-Guide-2018/dp/1457309289/ref=sr_1_3?ie=UTF8&qid=1518828443&sr=8-3&keywords=sat
https://www.amazon.com/SAT-Prep-Plus-2018-Strategies/dp/1506221300/ref=sr_1_1_sspa?ie=UTF8&qid=1518828443&sr=8-1-spons&keywords=sat&psc=1
Those are my tips for today, nothing further. There will be more tips in the next days, see you soon.
Valeu! Uma dica, se for postar em português o pessoal do @camoes e @lusofonia recomenda usar a tag #pt. Sucesso e boa sorte mais uma vez!!
Opa! Sou novato por aqui, então obrigado pela dica, vou aderir! Sucesso pra você também!