Amos, projectors are fed directly from the source, e.g. a TV tuner or a DVD player, they do not require a TV and are normally not connected to those.
Projectors are also not placed on top of the TV but rather at the opposite side (wall or ceiling mounted, or placed on a coffee table) and a screen goes into the place where you normally have your TV. Projecting against a white wall will work but a screen usually yields better results; good screens are expensive however.
As well note that you need to darken the room to get best results with a front projection system.
If you want to watch TV via a projector you need some kind of TV tuner that supplies the signal to the projector.
That said don’t you have a Satellite receiver now? That might be suitable, just need to ensure you can match the output of the SAT receiver to the projectors input.
Some general hints/advise:
If you are into DVD make sure the projector has a native 16:9 panel, progressive scan function and YUV input, if possible even a DVI-D input.
If you want to use it as PC monitor the resolution should be 1024x768 or greater, a VGA or DVI input is mandatory then.
If the projector cannot be mounted exactly on-axis to the screen you need keystone correction (projector too high or too low from the center of the screen) and/or lens shifting features (projector offset to left or right side), otherwise the resulting picture will look trapezoid.
Needless to say all those more advanced features make a projector more expensive.