I recently purchased a Nikon D50 with a Sigma 28-105 DG f2.8-4 lens. I love the camera and have taken some impressive pics with it. I have considered getting some filters to add some special touches to my photos. Finding photographic examples with and without certain types of lenses, has been hard for me. I have found alot of technical info on filters that I am, honestly, not educated enough to understand how they would affect a photo. Here are some things I would like to change in my photos...

Take long exposure photos (5 seconds?) of waterfalls in the daytime (ND filter?)...
Eliminate the dominance of red and orange colors in some situations...
Have more blueish color in photos (Have seen some photos with a blue tint that look great)

My question is, do I buy filters to do what I want or do I buy a software program to make the corrections I want. I know that software would not do what an ND filter would but maybe I could make the other corrections. Is there a basic filter package that would be good (but not too expensive) to have? Just looking for advice on how to choose what I need. Thanks for any input you may have and please forgive my ignorance on this subject.

Nathan