Today is the day you can upgrade your Apple iPhone to the new mobile operating system. Apple usually stick to a particular time frame so expect the servers to go live on or around 6pm GMT (10am PDT). As always, make sure you have a current backup of all your important data from your phone. Things rarely go wrong to be fair, but its much better to be safe than sorry.
In addition to the iOS12 release, Apple Watch owners will be able to update their watch to the new watch OS 4 which is also being released today. This brings with it new features and enhancements like the Walkie Talkie feature which I'm looking forward to playing with.
Heres a link to all things iOS 12 in case you want to read up on it before taking the plunge. All iPhone are capable from iPhone 5S and above, and watch OS is from Series 1 and newer (the original watch isn't covered I'm afraid)