Microsoft has started rolling out Windows 10 worldwide, but many Windows users--even those who have reserved their copy already--are still waiting to get the new operating system. That's good. It gives us time to prepare for the upgrade.
The most important thing you can do before upgrading is back up your entire system. Many of us just back up our important docs, but in this case you're better off creating a system image you can recover your entire computer back to. Both Windows 7 and Windows 8 have a built-in system image creation tool (if you know where to look), but it can take a long time to run. (Mine's been running for 2 hours!)
Before you back up, though, you'll probably want to clean up some disk space: Uninstall programs you don't need, empty the recycling bin, and so on, so your backup is pristine and you have enough room for Windows 10. (The new OS requires about 20 GB of free hard drive space.)
After cleaning up and backing up your system, you should be good to go, once the update is available for you. You can do an in-place upgrade, which will keep your programs and data or a clean upgrade using Microsoft's Media Creation Tool, which creates a bootable drive that will let you format your hard drive and install Windows 10 fresh.
You also might be able to upgrade to Windows 10 now even if your computer doesn't say the update is ready for you, Lifehacker points out you can use the same media creation tool and select the option to upgrade the PC now. Since this is an unsupported method, however, and some are reporting issues with the method, if you're not in a hurry to get Windows 10, remember that patience is a virtue.