If you have purchased a license or have access to one through your institution:

: Launch setup.exe and follow the Authorization Wizard to enter your legal license or trial code. ArcGIS for Desktop Student Trial software Instructions

If you have purchased ArcGIS 10.3 or have access to it through a subscription or your institution, follow these steps:

: Cracked versions are frequently unstable, leading to frequent crashes and data loss. Because updates are blocked, you miss critical security patches and performance improvements.