It is way more than 3 things. But basically, have VERY good grades in college preparatory classes. Take the PSAT in your junior year and answer the National Merit questions on the PSAT. You have to be a US Citizen or have applied for permanent residency to lead to citizenship. Your PSAT score will have to be in the top 50,000 tested in the country.
If you qualify, when you are invited by National Merit Scholarship Corporation to be a Commended Student or Semifinalist, name two colleges or universities you want to attend. In September of your Senior year you will be notified whether you are a Semifinalist. Then complete the required application and your school principal has to endorse you. Keep your grades up all of Junior and Senior years. Take the actual SAT and have your SAT scores sent to National Merit to confirm your PSAT score. Submit whatever other information they may ask from you. In February of your Senior year you will find out if you are a Finalist. In March you will find out if you are a Scholar.