I'd advise using a software manager if you have one installed, otherwise follow this guide to compile it yourself.
Download the source tarball, in this case it's the green download button that says ffmpeg-3.3.3.tar.bz2. Open a terminal and move to the directory containing the tarball (by default, it's ~/Downloads). To move to the downloads directory type cd ~/Downloads and hit enter (you said you are a newbie so forgive me if I'm dumbing it down too much). Now unzip the tarball by typing tar -vxjf ffmpeg-3.3.3.tar.bz2 and hit enter. Next type cd ffmpeg-3.3.3 and hit enter. Type ./configure && make and hit enter. If successful, you need to run make install as root to complete the process. If you don't have the sudo command installed, you need to type su root, then type cd ../whateveryourusernameis/Downloads/ffmpeg-3.3.3 && make install. If you have sudo, you should use this instead. Type sudo make install while in the ffmpeg directory. If you run into problems during the build, be sure to post the errors.