There are multiple things to consider when buying an HD TV:
What size TV should I get?
Lynn is going to be sitting about eight feet away from the screen. Leo says he'll want a bigger TV than he might think. At about 8 feet, Leo recommends 55-60" to get a true home theater experience. There's a this distance calculator to determine what's best.
Which is better, LCD or Plasma?
Leo says that Plasma screens are ideal for rooms that can be darkened. If he has a bright room, then LCD is best.
What about LED?
LED is the light that illuminates the back of an LCD panel. They're far more reliable and consistent than florescent backlit options. If he gets an LED TV, he'll want to get a backlit LED, not one that is edge lit.
What brand TV should I look at?
Leo prefers Samsung, but Panasonic, LG, and Sharp are also good options. Vizio is a lower-cost brand that makes decent TVs too, but it's not the best. He should go look at them though, to see what he may like best.
Should I get a 120hz or 240hz TV?
120hz is plenty, and he should certainly not pay more for this. Broadcast TV is 60hz. So anything over that will create a plastic looking thing called frame interpolation, which actually creates frames in between the actual frames. He can turn that off, though, but it can be good for sports and movies with lots of action.
What mode should the TV be in?
Make sure to set it to movie mode, not dynamic mode, for best results.
What about sound?
Leo recommends getting a home theater in the box, an inexpensive surround sound system. That will be much better than the speakers that come in the TV.