How do I activate VPN on my iPhone?

Which VPN are you subscribed to? iPhones don’t come with a VPN. Have you contacted the support team, for the service you are subscribed to, about the issue?

If you've been told to use a VPN to access your employer or school, contact your IT department or help desk for assistance.

If you want to use a VPN to access the Internet... just don't. It does absolutely nothing to secure your data or protect your privacy. It does exactly the opposite in many cases.