The SDHC (Secure Digital High Capacity) card goes in the 3-in-1 card reader slot on the left of the Mini 9. It's labelled SD-MS/Pro-MMC. That's the same SD as in SDHC (the card reader can handle both normal SD and SDHC cards).
Make sure you unmount your SD card before removing it or you could experience data loss. In the Ubuntu which shipped with my Mini 9, there seems to be a file browser sort of program which lists all the media on the left; you can right click on the card there and select "Unmount" to do this.
I'm afraid I don't know of a good beginner's guide to Ubuntu. I thought the Ubuntu community was supposed to be pretty good about that sort of thing, though; have you tried checking out the Ubuntu website? Alternatively, O'Reilly Press makes excellent manuals, including introductory ones; they might have something useful. Or just play around; if you're not superuser (using 'su' or 'sudo' or typing in your password when prompted to do so for temporary administrator privileges in the GUI), you can't mess things up too badly. (Terminal is found under Accessories on my Dell/Ubuntu, btw)
Black Mini 9 | 4GB SSD | [s:1kutak77]512MB RAM[/s:1kutak77] | [s:1kutak77]Ubuntu[/s:1kutak77]
RAM: Kingston HyperX 2GB 533MHz DDR2 CL3-3-3-8
OS: Gentoo Linux, ion3 WM