Your degree can be in pretty much anything. The head at the school I use to work at was originally a Maths teacher but had a degree in Computer Science.
You generally either know you 100% want to be a teacher and do a degree in a subject with teaching as part of it or you do a normal degree and then a PGCE year.
GTP is a bit different and I think you don't have to do any Uni work and it's just a full year in 1 school where as a PGCE year would normally include 2/3 placements.
PGCE is much more common. GTP you get paid a basic wage (something around £15k off the top of my head). PGCE did use to be paid (£666 per month I was told) but I believe you get minimal pay now.
An NQT could expect to earn anything from £16k up to £27k depending on your area, how good you are and the school.
The school I worked out would regularly hire NQT's on £23k to get the best of the bunch (£27k would only really be in London). A friend who did a GTP year got £30k for his NQT but that was in the centre of London in a struggling school desperate for good young teachers.
One word of warning, on your PGCE, NQT and NQT+1 (the year after your NQT year) be completely prepared to sack off your private life. I love to tease my OH with the whole 9am to 3:30pm for 39 weeks of the year but the truth is she has done 7am-6pm for the past 8 years as a teacher, regularly brings work home and often does evenings and weekends for special occasions. PGCE and NQT years are very tough and there is so much paperwork to do and things to keep you busy.