It took me a little while to get used to using VoiceOver. On this page are some of the commands I've used most often and some links where you can find out more about VoiceOver. There are also a lot of good instructions and info about VoiceOver on Apple's support site, I found it easiest to use the search to find content on VoiceOver.
The default VoiceOver modifier keys are control and option. You can change the keys (referred to as VO) in VoiceOver settings. To get there, go to System Preferences > Accessibility > VoiceOver. Then open the VoiceOver Utility.
In the settings, you can also make VoiceOver follow the mouse.
For more commands, check out the source material at the bottom of this page.
There are a few ways you can turn on/offVoiceOver on your iPhone or iPad:
There are a lot of useful gestured listed on Apple's support website, I included a link to the page in the source materials. I added links for both iPhone and iPad. At least the basic functionalities are the same in both devices, I've yet to encounter any differences. Here are the most common gestures I usually use: