Can I download the genuine version of Ubuntu from http://ubuntu.com? or do I need to make it genuine after downloading just like Microsoft Windows?
Ubuntu is always genuine!
Ubuntu is legally free in all conditions.
Microsoft refers Genuine to as 'None Pirate' version of its Operating System (Windows).
It would be legal if you install it on as many of your computers or even to let your friends or family install it on theirs.
After Edit: You mean 'Activation'. No since there is no licence granted to individual users unlike Windows, so you are not required to activate. You don't need any CD/Licence keys.