If you go for one of our recommendations (ExpressVPN, CyberGhost or NordVPN), it will be extremely simple. Once you have subscribed, all you have to do is go to the Apple Store on your iPhone / iPad in order to download the application.
Then enter your credentials and off you go! No special configuration is required. However, afterwards, it may be interesting to modify the protocol used according to your needs (by going to the VPN application settings).