You need to have gone to an MD/MBBS program that is certified by the ECFMG (education commission of foreign medical graduates). After completing basic sciences (first two years of your degree) you’ll get a clearance letter from the university. You take that letter and contact ECFMG. You apply through them. There is a process and fees for the exams.
You can take USMLE Step 1 or Step 2 (CK or CS) in any order you want. Step 3 can only be taken after you’ve cleared step 1 and 2.
If your school is not ECFMG certified (and not inside the US) then I’m afraid you may not be able to register for it.