norfleet... you asked a mouthful!
Show and No-Show is just a way to display or hide geometry. Most users will no-show construction elements (points, lines, surfaces) when they are working with a solid. But when modifying the solid, you probably want to re-display the construction elements you want to change.
2D and 3D really have nothing to do with SHOW/NOSHOW. 2D is just an easy way to create 3D elements that are in a 2D plane. These points/lines/curves can be modified in either 2D or 3D modes.
Tips on how to modify a model? I'd suggest you find someone you work with who is a little more experienced with CATIA, and use that person to mentor you through the learning curve. V4 can be a little difficult at first, but it will eventually make sense.
Here's some tips that might help:
To show everything:
click NS, yes swap, key *SPC, click EXIT
to hide everything except for solids:
click NS, key *SPC-*SOL, click EXIT