Cool - so happy you tried it.

The easiest way to install programs (like firefox) is just to open a terminal window and type the commands `sudo apt-get update` and then `sudo apt-get install firefox`. The first command will update the system's repository lists, and the second will install firefox. 

You'll be amazed how much software is available to install this way. 

To do it through the GUI... I can't remember if Lubuntu has a "Software Center" application. If not you'll have to use the program called "Synaptic" somewhere in the settings or applications menu. Open Synaptic, search for the program you want (like Firefox) and it should turn it up. Synaptic manages the dependent packages for you; all you have to do is find the main software package, select it and hit apply, and it does the rest. 

Usually though I just google "install such-and-such a program from the Ubuntu command line" and find commands to cut and paste. (Because Lubuntu is based on Ubuntu, anything that works for the Ubuntu command line will work for you as well. Same for Mint.)