Ubuntu is a the most popular free and opensource operating system in the world! It is user friendly and packed with features. You get to choose from many different flavours to fit your needs.
Step 1: Choose a flavour.
Every flavour of Ubuntu has it's own user interface to cater for the needs of various users. The flavour you chose will be determined by your needs and preferences.
Ubuntu (unity) is the flagship flavour of Ubuntu. It uses a modern User Interface, with the focus on ease of use. Perfect for beginners.
Kubuntu is Ubuntu using the KDE interface which is a more tradition User Interface. Perfect if you prefer a traditional desktop experience.
Edubuntu is the same as Ubuntu with unity but includes educational software for kids of all ages. Perfect for classrooms, students and teachers.
Xubuntu and Lubuntu are lightweight versions of Ubuntu running on the XFCE and LXDE desktop environments respectively. Lubuntu is even more lightweight than Xubuntu, but packs fewer features. Perfect for older computers.
Ubuntu GNOME uses, like Ubuntu (unity), a modern User Interface aimed at ease of use. Perfect for users who want a modern UI, but do not like Unity.
Ubuntu MATE uses the MATE User Interface which is based on the old GNOME 2 interface. Perfect for users who prefer GNOME 2 over Unity.
More on flavours here.
Step 2: Make a disk or bootable flash drive and install.
Also see the Ubuntu installation guide.
Step 3: Enjoy!
That's It! You can now use Ubuntu and install any programs you want from the Software Center.
Once you are done, come say hello!