1. Do lots of research: tedious and time consuming, but it's worth understanding the jargon. LED sets tend to be the lowest energy consumption. Don't bother with plasma sets.
2. Don't buy from Sainsbury's as their prices are consistently higher than places like Currys or Comet, even though they claim "special offers". Check on-line if you don't believe me.
3. Go for a trusted, probably far east, big-name brand as these companies have a global reputation to maintain.
4. Decide on the size, if possible cut out some paper so that you have a realistic idea of how big the actual screen size will be.
5. You can get some basic info. from the "Which" web site, but it will try and get you to buy their guide to TV if you sign up for a life subscription to Which magazine!
6. Visit somewhere like Curry's in Catford and you will realise the enormous range of TVs available. Staff are sometimes helpful and then sometimes will knock you sideways with tons of technical info. Don't be afraid to ask lots of questions. Remember that modern TVs can also be used as computer monitors, for games, photos and so on. Smart TVs are probably the future as they have things like BBC i-player available.
7. Don't bother with 3D.

Further advice available: just ask!