In my opinion, the most important thing for beginners to know when switching to a major Linux distribution is that the usual way of installing software is different on Linux.
The Windows approach is that the operating system comes with a bare minimum of included software, and for anything else, even somewhat "essential" things like office suites, you need to buy, download and install them separately, using separate installers.
On a Linux distribution, this is not how it goes. Virtually all the software you will ever need is provided by the operating system as part of their official update mechanism, which is run by their package manager.
This includes several office suites, several photo editors, several video editors, several browsers, several instant messengers, several text editors/IDEs, and so on. There are literally about thirty thousand software packages provided by Ubuntu, which you can install at any time from its own packaging system. It is very rare to have to install something that isn't provided by your distribution.
This is even more the case with Ubuntu, which even provides several closed-source applications such as Skype and Flash - which on some other Linux distributions would be exceptions to the general rule (Skype requires you to enable Ubuntu's "partner" repository). In Ubuntu, even these, and other closed-source applications like Adobe Reader, are available from its repositories.
In a major Linux distribution, installing software the "old fashioned" way is almost always a mistake, unless the software is rare enough not to exist in your distribution's software repositories.
If you find an online tutorial telling you to download PHP as a .tar.gz from php.net, that is the wrong way to do it. If you find an online tutorial telling you to download a .deb file from a website in order to install, say, Filezilla, you're doing it wrong.
If the software is provided by your distribution, which if it is open source software that is reasonably popular and useful it will be, then the right way to do it is via your package manager.
Users, especially beginners, should be especially cautious about installing any software that doesn't come from their distribution's packaging system. This includes, in Ubuntu, the use of PPAs. Unless you are doing something quite unusual, and you accept the responsibilities of having non-distribution-maintained software on your system, find the equivalent in your distribution's repositories and install that.
That is, in my opinion, the most important thing that newbies should understand when they are new to a Linux distribution.