he he lotsa question.... ok deep breath... here we go;)
First check freeview coverage for your area
http://www.freeview.co.uk/availability/
if you have coverage (BTW my postcode tells me I haven't but it still works fine!!) then you will require a set top box (STB) these will connect to your current tv by either the normal aerial socket, or for better results by a scart socket, if connected by aerial socket you will need to tune a spare channel on your tv into the STB, if connected by scart then you sellect the AV input on your TV. (basicly they connect in to the TV in the same manner as a video recorder does)
Once it is all connected you will still be able to get your normal 4 channels in the same way you do now untill the time the ananlog signal is cut off in your area (2012 at latest so little way to go yet)
Freeview boxes do have teletext, but not the same teletext as you get on your current analog setup, but you will still be able to get teletext from the analog signal on the normal channels just as you do now.
You may need a new aerial, best thing is to try it and see if it works.
if you are happy with your current television set then no need to get a new one.
I would suggest that as freeview boxes are available from most large supermarkets for less than �20 now to simply buy one and see how you get on.
Hope that helped
Mark