If you've got an iPhone 4, iPhone 3GS, iPod touch 3G, iPod touch 4G or any iPad, you can download and install Apple's iOS 5 update right now. All you need to do is open up iTunes (make sure you have iTunes 10.5 installed) and hit the Update button. While the update is downloading, you can familiarise yourself with the new features. Here's a video that tells you everything you need to know.
Once you've updated and synced your device, you can configure the Notification Center, set up iCloud, and set up iSync on your device. Unless you have an iPhone 4S, you can't use Siri, or at least you can't until hackers manage to port it to iPhone 4.