Your standard TV is in 4:3, meaning that for every four pixels (little squares of light that make up your image) going across the TV, there are three going up it. If you happend to own a fancy/newer TV, it could very well be in 16:9 (Widescreen). Major motion pictures are shot in a number of different formats, but 2.35:1 is considered Letterbox. The closer you get your image to 2.35, the more professional it is going to look because, thanks to movie theatres and DVDs, we are all used to seeing films in Letterbox and Widescreen.

My suggestion at this point, if you want to go Widescreen or Letterbox, is to storyboard in 16:9, shoot in 4:3, and then fix it in Post. It will be helpful, if it is possible, to put a guide around your camera so you can make sure that what you want in the frame is within the 16:9 guidelines. If you don't have a guide, you are far more likely to accidentally shoot a scene where one of your character's heads is not in the frame. I cut a rectangle out of an index card and taped it to my camera's viewfinder when I shot 2.35:1, so it can be done for under 5 cents (if you have a calcular with which you can do the calculations*).

*Let us assume that your camera's viewfinder is 4:3. Let us also assume that it is in perfect centemeters. Since you want to keep your horizonal length the same, you will need to determine how much of the vertical space on the screen you will need to cover up. Your calculations will need to be as follows:

4cm/x=16/9 Multiply both sides by 9
36cm/x=16 Multiply both sides by x/16
36cm/16=x => x=2.25cm

Thus, your screen needs to be 4:2.25, and so you will need to cover up .75cm of the vertical area of the screen.

NB: You must devide .75 in half because half will be covering the top and half will be covering the bottom of the vertical space so that your image is still in the center of the frame. Thus, .75/2 or .75*.5=.375cm which is approximately 4mm.
(Who knew that Algebra would come into play in filmmaking?)