You'll first need to buy the Snow Leopard disk from the Apple Store: http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
Back up your Mac to at least one external hard drive.
Once you've installed Snow Leopard, run at least Address Book, iPhoto, iTunes, Mail, and iCal to have it update your Libraries to the current format.
Also update it to 10.6.8 using Software Update. Also see if you can update iPhoto in the App Store. Yours may be too old for an update.
If that is running OK, then you might want to back that up, too. If you decide to revert to the older OS, you will be much happier with Snow Leopard.
I'm not sure where you got the advice not to upgrade from Leopard. Snow Leopard was way faster than Leopard.
Before you upgrade to Yosemite, you'll need to max out the RAM. 2GB is the advertised min, but it isn't enough. You can put in up to 8GB, which is what I would do. It will run everything better with that much RAM, even if you don't stay with Yosemite.
There are a lot of changes from Leopard all the way to Yosemite.
Some big ones: While they restored contacts and calendar sync with iTunes, most everything is done through iCloud, not plugging into iTunes.
Snow Leopard and prior could run PowerPC (the old processor Apple used) applications, but they won't run anymore. If there are PowerPC programs you use, then you'll need to see if you can upgrade those to a newer version. You might consider Cloning your Snow Leopard disk so that you can boot into it after upgrading to Yosemite.
When you are ready to upgrade, Go to the Mac App Store (Apple Menu) and find Yosemite.
If you keep a clone of your Snow Leopard on an external, you can always revert to that if you don't like Yosemite, eventually cloning it back to your internal.
For Cloning, I use Carbon Copy Cloner, but others also recommend SuperDuper.
