1. DON’T INSTALL HIGH SIERRA ON OLD COMPUTERS!:
Although you might get a message from your Mac App Store telling you that the latest operating system, macOS Sierra 10.13, is available for free download, think hard before installing it.
This system is designed expressly for Macs with an SSD (a Solid State Hard drive). These are fitted to all current laptops and to iMacs if specified upon purchase — and after coughing up a lot of extra dollars. iMacs also can have Fusion Drives which are a combination of both modern SSDs and the older regular hard drives (HDs) that have been with us for decades.
If your Mac is older than 4/5 years forget about it unless you have lots of RAM. By lots I mean at least 16GB. I have had so many calls from people on older machines who have installed it and their beloved machines are now running so slowly. There isn’t any fix. Popping lots of RAM helps a lot, but is expensive at $200-$300. Just stick with Sierra 10.12.
One last thing: if you do install High Sierra be very careful. During the install it asks you to login with your Apple ID and then prompts you to put your Desktop and Documents folders into the iCloud Drive. DON’T DO IT! This tries to upload everything on your Desktop and in Documents up to Apple’s iCloud. This isn’t viable with our lousy internet and everything grinds to a halt… Untick the box that is ticked by default, which I think is criminal.