How Can I Improve Picture Quality On A Sharp Aquos Lcd Tv?

It’s a source problem, not a problem with the television. If you’re watching standard definition programming, the picture is going to look grainy and washed-out. High-definition content will look much better. Does the television have a QAM (over the air) tuner? If it does (and it almost certainly does), connect an antenna and scan for channels. Depending on your location, you should be able to find at least one, unless the TV is in a basement or something. An over-the-air HD channel will give you a good idea of what the TV is capable of displaying, given quality content such as would be provided by a blu-ray player, upscaling DVD player, or high-def programming from your cable provider.
For the most part, cable is going to look like crap until you get a HD box from your cable provider and connect it to the TV with component cables or an HDMI cable (of the two, the HDMI connection would be better.)

Probably because you are looking at standard def, crappy cable.
Get HD and a blu ray player. No point in getting a HDTV if you don’t have HD material to feed it.