Really, you should not do it, but sometimes it’s a must. The safest way to install new software on your Linux computer is to use the software repositories that are available for your distribution. This helps assure that the program you want to install is stable, and virus free.
Sometimes though, there may be a program that you want that isn’t available any other way In this case, please do some research! Know your source before doing the install!
There are a few ways to do it, and all the methods are fairly easy to do. Some developers will simply provide a download of the .deb install file. Just download the file for your distribution, and open it with the Package Installer
If you use Python software, you can use PIP, or if you’re crazy like us, you can download the source files and compile the software yourself.
Software you may wish to install 3rd party, includes packages like OwnCloud and TeamViewer, which don’t always show up in the repositories but are known safe.
Point being, there are lots of ways to install software in Linux, but please, for your own sanity, until you are really comfortable with Linux, just use the Software Manager!