Identify the field in which you want to become a scientist. You could be scientist in Maths, Physics, Biology, Chemistry, Economy, potentially every field in which there is scope of research. Identify the are you like and then look out for good colleges in that particular field. Once you decide a few universities, see their admission process. Is it via an exam or interview. See the syllabus of the type of exam and then prepare for it. Look out which books could be helpful for that particular exam and start preparing.