Apple has released iOS 8 to the masses
iOS 8 is compatible with the iPhone 4s and the iPad 2 onwards. As with previous systems, if your device is the oldest one on the list, so in the case of the iPhone 4S, you may want to hold off upgrading until you have heard from other people about performance and reliability on these devices.
Checking the amount of free space available is really recommended. Last year when iOS 7 was shipped the space requirements where approx 730MB to download the install image direct to the device and then approx 3.1GB to install the update.
If you are no where near this, then it might be time well spent removing those apps you never use.
You can reduce the amount of space required by installing the update via iTunes on your computer rather than over the air direct to the device. If you are really short of space, then removing your music or photos temporarily maybe of some help.
Updating iOS has always been very reliable, but there are bound to be a few people out the millions who upgrade who run across issues. So get a backup in before you upgrade.
Backing up is easy and there are no excuses. You can either backup to iCloud, assuming you have enough space, or backup via iTunes on your computer.
Apple have a nice tech article here on how to backup.
Check all your apps are up to date. This goes for the version of iOS 7 you are running and also your apps.
Assuming you have done all this, get ready to enjoy iOS 8.