The best thing to do is go to a store that has iMacs and MacBooks on display and try one out. Some of the big box stores in the major cities are Apple authorized dealers, and they will have displays of Macs and employees to help.
You might want to get a MacBook, because the displays are large and and easy to see, and the MacBook, if course, is much more portable. But an iMac will be about the same price and have, of course, a much larger screen and a full size keyboard.
If you don’t want to go to a store and are sure you want an iMac or a MacBook, you can order online from a big online store like amazon.com or even directly from the Official Apple Store.
When you buy from a store, there is little reason to get the extended warranty they will want to sell you. It’s best to get AppleCare directly from Apple or from the store at the same time you buy the iMac. AppleCare will let you take the Mac to an authorized dealer for help.
When you unbox the Mac you get from a store or from online, you’ll find quick start instructions. And when you start it up for the first time, it will walk you through adding your personalization, setting up your internet connection (which you should have ready to go from the phone/cable/satellite company) and getting started with using it.
You also need to get a book or two to go with the iMac or MacBook. Apple ships all Macs with documentation on them, but it’s very helpful to have a book to work through. Some of the best are Switching to the Mac: The Missing Manual, Mavericks Edition: David Pogue (Mavericks is the name of the newest Mac operating system.) Get that one if you are moving from Microsoft Windows to a Mac. If you’re not moving from Windows and you are starting from scratch with a Mac, get OS X Mavericks: The Missing Manual: David Pogue. A big book store will have those on the shelf, or you can order online.
And once you get the iMac set up ad running, you can check out the videos that Apple has for learning how to do things. Go to Apple – Support – Videos