AutoCAD is the "industry standard" for CAD programs and the most popular for drafters and designers working today. It has been around since 1982 and gone through many changes. In the most recent versions (since 2010) there have been annual updates, but the changes for the average user are minimal. The current version (2017) is so similar to AutoCAD 2010 that you can easily switch from one to the other.
To start with, AutoCAD is a very expensive program to license. You can't actually 'buy' AutoCAD, but you license it from Autodesk. Which also means you can't sell it when you're done with it. That keeps you from finding used versions for sale. To purchase a fully licensed version, you would contact your local Autodesk reseller.
What you can do instead is get a free trial from Autodesk. The current version is AutoCAD 2017. People can download a 30 day trial version or a 3 year 'student' version.
Free trial: http://www.autodesk.com/products/autocad/free-trial
Student Trial: http://www.autodesk.com/education/free-software/autocad
If you get the free 30 day trial, this will give you enough time to go through the tutorials on myCADsite.com. The student version is ideal, but you need to meet certain requirement to get this version.
When you're ready to purchase the real thing - get AutoCAD license on Amazon.
AutoCAD LT is a stripped down version - basically no 3D capabilities and no programming. It is more affordable for anyone wanting to purchase it. Great from home businesses or companies that may have the full version, but need additional 'seats' (liceneses).
From there, Autodesk has built 'industry specific' programs that have expanded from the AutoCAD base to add more robust features.