Today is the day. Apple will release iOS 8 to the public as a free update at approximately 1pm EDT. In my opinion, this is the biggest iOS update in years, so you're going to want to download it, but you're going to want to do some things to prepare first.
The update will roll out as an OTA (over-the-air) update, but you will want to create a backup beforehand just in case. It never hurts to have a backup. Of course, an iCloud backup is good, but when it comes to big updates, I also suggest plugging your iPhone into iTunes and doing an old fashioned backup on your computer.
Evaluate your device before you move ahead with the free upgrade. Last year iOS 7.0 severely slowed down iPhone 4's, and although I'm sure Apple learned from last year, the oldest models that are supported don't tend to work as well with the new software as newer ones. This year, the oldest devices that support iOS 8 are the iPhone 4S, iPod Touch (5th gen.), iPad 2 and iPad Mini (non-retina). If you own one of those devices, I suggest waiting to see what the feedback is from users who have upgraded. Give it a few days. If you have an iPhone 5, iPad 3 or newer, you should be fine.
This might seem like something you shouldn't have to worry about, because Apple products are supposed to "just work", but keep in mind that an iPhone 4S is 3 years old and it's running on the A5 processor, which originally debuted in March of 2011. In mobile device processors, 3.5 years is a long time. Apple has a difficult dance to do when it comes to deciding which devices to support. If they cut off support for new OS updates too soon, people will complain that Apple is purposely leaving their device out in the cold so they are forced to buy a new one. Some people will even wrongly use the word "obsolete" in this case to describe what Apple has done to their device. If Apple continues to rollout updates to devices for too long and the devices are too old to handle it well, then you'll be left with a device that's slow and clunky, and people will again complain that Apple is doing it on purpose to make them spend more money on a new device. Neither of these things are actually true, but Apple can't win in this case. They want you to have a good experience, so you will buy a new device. Either making your device "obsolete", or purposely making it slow, are not things that would make the customer have a good experience, and would probably have the opposite effect. If you think Apple is screwing you on purpose, you're probably not running out to buy another iPhone. You're likely going to be looking at an Android or Windows phone. Apple doesn't want that.
Go For It
Get yourself on Wi-Fi, so that you're not using up too much of your precious data plan, plug in your device to a power source and watch the little while Apple and status bar on your screen as your device updates.
Apple has included a new "Tips" app in iOS 8 to help you learn about iOS 8, but nothing beats exploring for yourself. The best things about iOS 8 are mostly on the third party developer side, so you're going to want to update your apps and look for new ones that support some of the great features in iOS 8, like extensions and customizable keyboards.