Firts of have you got Dark BASIC Pro, if yes then look at some of the tutorials or source code in the code base for games like space invaders and pong as they are quite simple. Then start work on the type of side scrolling game you want (sonic the hedgehog style, street fighter or one of the dozen space shooter games on the Dreamcast). There is a reall good book you can buy of amazon for complete beginners, thats were I'm starting from.
Good luck
Oli
Now have, P4 2.8Ghz, 80gb hardrive, 512ram, Gforce 5200 128mb! Soon to upgrade to 1024 MB RAM and X800 XT ATI 256MB card! (yay!)