I'm just a guessin' this is your 1st NIT
I'm sure I will be forgetting a few things here...
I suggest a couple of sd cards...16gb internal and 32gb external.
(Do not remove back cover while NIT (nokia internet tablet) is running with internal sd card in.)
Format them both for nit use...
grab the nokia software updater....(assuming you use windows)
flash (install) latest diablo for n800 (rx34) firmware...(essentially the latest operating system).
While on your n800...go to the download page for 2008 programs here at maemo.org and download rootsh onto your NIT....it will enable another repository at the same time.
Look up ASUI here.
Look up Telescope here.
Look up Console tools (by penguinbait) make sure you pay attention here this is where the rootsh program comes in handy.
Use console tools to essentially make a copy of your pristine operating system onto your 16gb card (saves system wear and tear and means you can always make backup copies of your entire operating system and save it to another sd card or your pc )
Once you have this done start-up into the sd card...and upgrade diablo to CSSU and Diablo TURBO wheeee! and not to forget check out www.gronmayer.com/it and install ALL those repositories. then you will have a ton of apps...As well you can install Easy Deb which is purdy good too.

OR if all that sounded like martian at this point....just use your N800 and go to the maemo.org download page find the skype install and there you go. But I highly suggest you optimize your system and battery life by read-read-reading and doing at least some of what I originally suggested.
Good luck and shout if you need help.