How: For Intel Macs: Turn on your Mac, then immediately press and hold the D key on your keyboard as your Mac starts up. Release when you see a progress bar or you're asked to choose a language. For Silicon Macs: Turn on your Mac and continue to press and hold the power button as your Mac starts up. Release when you see the startup options ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.