what you need depends on what you want to do.
Well, first of all you need programs to extract the archives and create entity lists from the maps. VPK tools does nicely, but I think the SDK contains some newer, and maybe better programmes for that purpose.
Then you need a good text editor for python scripting, dialog editing, and all the other text files within the game. I use Notepad++
If you want to work with the textures you need converting tools (vpk tools again, and I think there is another one in the SDK) and photoshop or Gimp. I don't know if there are other graphics programms that allow to save in DDS format.
Honestly, modding bloodlines is too complex to get you started in one post.
As Feral said, if you have a more specific question, ask. I don't bite (at least not Kindred because of bloodbond)
P.S. Bloodthief, go home.
Tuesday is Soylent Green day.