How to install Ubuntu software when you're a (future) ex-Windows user!

Remember that some day you'll have to remove this piece of software that you installed, so always use a removal method identical to your installation method.

Therefore, use the following priority for installing software on Ubuntu:

For beginning users: first 3 months, up to a year of using Ubuntu
Install/remove from the standard Ubuntu repositories using the GUI of the Ubuntu Software (Center).

Click the dash, type software, click the Ubuntu Software(Center).

Opening Ubuntu SW Center

The Ubuntu Software (Center) opens:

Ubuntu SW Center Start

and you have a ton of application categories on the left to choose from. Or type the name of the software in the search box in the top right corner (which is what we'll be using)

Crap software

I'm as amazed as you, but there is indeed crap software for Ubuntu, so just click the crap you want, click on "Install", wait a bit and done!

To remove software using this method, click on the "installed" button on the second screen shot, click the crap you want to uninstall and click the "Uninstall" button! Easy-peasy
(Intermediate users, 6 months-1 year of experience)
Use the TUI of aptitude
Press Ctrl+Alt+T to go to a terminal and type:

sudo aptitude
and press Enter

If you get aptitude: command not found, type:

sudo apt-get install aptitude
to install aptitude and wait until nothing moves any more and then type:

sudo aptitude
to start it.

screenshot aptitude

This is still kind of familiar: you can use the mouse, but it's like you're back in the 90's before the Internet was invented. And now comes the first hurdle: RTFM for aptitude!

This is important! aptitude can let you do more advanced things, but is a back-stabbing servant!
Now on to more advanced stuff: (Use only when directed by giants, that is: knowledgeable people on this site having >1000 reputation + 1 or more silver badges)

Press Ctrl+Alt+T and type:

apt-get install szPackageName to install
apt-get purge szPackageName to completely remove
apt-get remove szPackageName to remove the application, keeping its configuration files. (meaning: you might want to reinstall this sometime later and you just spent a few hours configuring the damn thing and don't want to lose the config!)
Really advanced stuff: (Use only when directed by immortals, that is: knowledgeable people on this site having >5000 reputation + 1 or more gold badges)

a. Download and install a .deb file: use dpgk --install szPackageName and dpgk --purge szPackageName and dpgk --remove szPackageName to install, completely remove and remove without config files.

b. Install a PPA: clearly follow instructions. if anything goes wrong, copy-paste the error and report back to the immortal!
'Just download and build from source!!!'

Probably a developer telling you this and he knows nothing about installing and maintaining a stable system without any problems, but has the coolest stuff on the planet!
Depending on your point of view to be embraced with love or to be avoided like the plague.