If you have installed MikTeX, TexLive or MacTex, your question is answered here or here. If not, you need to install TeXLive or MacTeX.
I think your problem is that you haven't installed LaTeX but just an editor.
You have to install LaTeX. There are different solutions how to install LaTeX, depending on which operating system you use.
Open an editor (Strg+Alt+T). Type following in:
sudo apt-get update
sudo apt-get install texlive-full
You can also download TexLive for Linux on your own:
sudo mount -o loop texlive2015-20150523.iso /mnt
Download TexLive here and just install it the usual way.
Install MacTeX from here.
That should fix your problem. Your problem is, that you haven't installed the libraries for LaTeX, not installed LaTeX at all. You have just installed an editor, a user interface that helps you editing and writing LaTeX code. But you have nothing that compiles your code to a PDF.