You need a graphics card with two video ports (either two dvi, two vga, or one dvi and one vga). You could get a vga or dvi split cord (forgot the technical name for it) and plug it into one video port but the two screens you hook it up to will display the exact same picture. Ppl usually only use it to display power points on a projector while watching their computer screen.
If you want to dual monitor then you need a graphics card with two video ports. Just make sure you know which ports you need (check what you have on your monitor).
example: This graphics card
has two dvi ports. So you could say get two of these monitors
and plug each one into a different dvi port on the graphics card.
Then you just install nvidia control panel (the software that comes with the graphics card) and enable dual monitors. If you have the screens where they are on the wrong side and you want to switch them, then you right click desktop -> properties -> settings and drag one of the monitors to the other side of the other monitor and you can change the primary monitor (the one with the task bar at the bottom) through this window as well.
Hope that explains it