Your questions are fundamental to the fountain pen world. For this type of pen, known as "cartridge/converter," there are two ways to fill it. One is through standard international short ink cartridges, of which I believe 2 come with the pen. They are easy to install. You can buy them in any number of ink colors through any number of vendors, including amazon; just remember that the standard international short variety is what will fit this pen.
The second way is through use of a "converter," a small piston operated device through which ink is drawn up through the nib of the pen, stored in the converter, and delivered through the pen's feed while writing. This requires a bit more thought than just installing a cartridge, because you have to obtain bottled ink. This, of course, is the fun part for many of us fountain pen enthusiasts, but may or may not be to your taste. As for operating the converter, this is easy: go to YouTube and search "using a fountain pen converter" for a tutorial.
Hope this helps!