Once you’re set on colleges, take a look at their deadlines and start registering for the mandated exams. The General GRE is pretty much a standard for all STEM courses. Business and management courses will need GMAT. Specialized courses like pure sciences, literature and mathematics might need a subject GRE but few universities have begun phasing them out recently. Except very few cases, which differ with each college, you will almost certainly need to take a language test, either TOEFL or IELTS. Go figure.