If you have little to no knowledge of modeling clothes and making them Poser ready you might want to a search for Dr. eep's tuts on the Setup Room, joints, rigging. Read through them, play, and get a feel for what all is involved. As far as modeling apps, try the free ones, such as Blender, wings3D,Anim8tor, first. Try the demos of the more expensive ones. Find the one that suits your likings before investing a lot of money.

I must remember to remember what it was I had to remember.