Just start the game with -console added to the shortcut command line. Press tilde (~) to open the console in game, and use these cheats to do various things. Note that the console is very friendly, and if you're looking for a particular code just press a key like 'g', and all the commands that start with g will display. Press up and down on your arrows to scroll through those commands, and enter to activate the selected one.

Code - Effect

freecasting - no blood loss when using disciplines
blood # - replace # with the number of points you wish your blood pool to be at. For example 'blood 4' sets your blood pool to 4 points.
giftxp # - give # number of xp points
give item_a_xxxx - give armor of type xxxx
give item_w_xxxx - give weapon of type xxxx
god or godmode - enable godmode