How hard is the new GCSE, really? Watch
However, in terms of English literature/language and subjects like History and Geography, the content you cover is quite simplistic and easy to remember. Obviously more so if you revise. I think it's just the vast array of things you need to know that's the tough part. It's rather unfair too - 10 poems (if I remember right) for you to use 2 in the exam for literature. Ugh.
Maths and Sciences are similar. Lots of content that you don't need vast amounts of detail on. They were more varied in terms of difficulty though. Too much of a focus on equations for me. I found maths very difficult - but always have.
In regards to revision - the earlier the better. I'd prep your cue cards and posters long before exams so they're there for you when you need them. I'd also plan as many questions as possible and do past papers. Although the last 6 months are most vital for revision with past papers and plans. Other revision is useful throughout. I started really revising (as in a little every day) in the last 5 months.
It's not as difficult as you first think though. Practice makes it all a bit better.