Also keep in mind the App Store will check for system compatibility, so if you can download it to your system without error, then it should install just fine.
Next, ensure your system is not experiencing any major problems, which include regular freezes or crashes of applications or the entire system, or the inability to log in to a user account, enable sharing services, or establish hardware connections to peripheral devices or networks.
Before you make major change to your Mac, make sure to back up your Mac.
It doesn't matter if you use Time Machine, copy files to an external hard drive or clone your Mac, just make sure things get backed up before you do.
Select the hard drive you'd like to install it on (for most of us, that's the only hard drive the Mac has), and then let the installer do its thing — your Mac will restart and begin the upgrade process.
For the most part, installing Yosemite is a hands-off process.
Now that Yosemite has been out for a bit, most application developers have updated their apps with new versions optimized for the new operating system. So the best thing to do is to check with each individual developer of the apps you rely on the most to make sure you're using the most up-to-date version, and that they've tested their software to work with Yosemite. Apple's heavily promoting Yosemite right now so you shouldn't have any trouble finding it and getting it, but you can always click here and make it easy on yourself.
It's a free update, so you don't have to worry about getting charged for it.
If your Mac can run OS X Mountain Lion then it should run Mavericks, and in its announcement today, Apple reiterated this includes the mid-2007 i Mac, Mac Book Airs from late-2008, and the Mac Mini from early 2009.The installer can be copied like any other file or application - just drag it to the target disk.