I'm actually an United States citizen who has been living and being educated in Malaysia. So currently my family decided to send me back to the States to continue my future studies. Some people told me that I need TOEFL/SAT results to prove my English but some of them said that I don't need it because I'm a citizen. I'm kinda lost now... What should I do to prepare myself before going there?
As Nate Eldredge already mentions in a comment, there will be a statement somewhere on the application page that specifies who needs to provide language scores as part of their application and who doesn't. Since schools receive large numbers of applications, there isn't really any leeway for special cases: Just follow the letter of the application requirements. If this requires that you to take the TOEFL or SAT, then that's what it is -- whether you think that's fair or necessary or required is a question that's entirely unimportant.
There is no general rule on it. Each university may have its own requirements. You need to follow admission requirements which are often clearly declared on the universities' website. Otherwise, send an email and ask admissions office about your specific condition.
However, depending on universities' policies, some candidates who have not lived in an English speaking country may not need to sit English tests. As an instance, if applicants have been studied in an institute in which the language of instruction is English or have worked in a company in which the main working language is English (such as international companies, foreign affairs/business/translations/education departments, etc.) for more than two years; these candidates usually do not need to sit any language tests. They need to prove this fact, like the language of instruction should be written in their graduation documents, their manager/ instructor/ supervisor should write a letter to the university and clearly declare that candidate have been working in an English environment, etc.
So, the point that you are a citizen of an English speaking country does not really mean that you do not need to sit any required tests. It is important that you have lived in a non-English speaking country. Language of instruction is very important. If you have studied in local schools and in Malaysian, you may need to prove that you have enough language skills by sitting a test.
Please also note that some programs require test scores of GRE/SAT etc as part of their assessment of all the candidates, even native English speakers. These tests are not language tests and mainly focus on assessing candidate's skills in mathematics, logical reasoning, etc. Sometimes candidates need to sit both a GRE/SAT test (depending on which program they are applying to: undergraduate or graduate degrees) as well as an English test and both are part of their admission documents.