It is not possible to provide generic advice about all international education systems, but here is an explanation about UK qualifications - this may help you identify the equivalent. In the UK GCSEs are sat at 16, and A-levels at 18, A-levels are the traditional entry requirement for the UK University system.
You can enter your qualifications on the GCSE and A-Level pages by selecting 'Other' from the 'Level' dropdown list. You will notice that when 'Other' is selected a box allowing you to provide the name of the qualification appears. You should complete the GCSE boxes and A/S-Levels and Other Equivalent Examinations sections.