Hi,
FP1 introduces a lot of new concepts in maths which you won't have come across at all in GCSE or normal maths A-level. Personally, I enjoyed learning these topics, and I found FP1 much more interesting than any of my normal maths modules.
Once you get used to the material the exam questions tend to be quite predictable. (I did edexcel, it might be different on other exam boards. I do really recommend the edexcel course though because the textbooks are great for self teaching- loads of examples.)
FP1 only requires C1 and C2 knowledge, although most of it is unrelated to C1 and C2 and I was fine doing it before I'd finished C2.
The important thing for self teaching maths is just to do all the questions in the book. Don't do halves of exercises and follow up every question you can't do. When you're finished the teaching- this should be around easter time- just focus on doing past papers, again, following up any questions you can't do.
You'll have to do two more modules for an AS, which can be done in either year. If you're interested in physics do a mechanics module- M1 is mostly covered in physics AS so will be really easy, and M2 is also linked to physics.