The entry requirements vary between countries and even universities. Read the prospectus of your shortlisted universities to know the requirements.
If you are applying for an undergraduate course, you must prove that you have complete your secondary education and have the required grades as indicated by the university.
If you are a non-native English speaker and want to study in an English-speaking country, you will need to take some prescribed English language proficiency tests such as IELTS or TOEFL and get the required grades.
Apart from this, countries have different eligibility requirements but fret not there are more similarities than differences.
Countries such as the UK, U.S, Canada, Australia and Germany require proof that you have enough funds to pay for the course that you want to study. This can be your bank statements or copies of your income tax returns. You should have the required band scores in your IELTS exams to get a student visa to study in the UK, US, Canada, Australia or Germany.