I'll tell you about my school, but some are different. In Scotland you usually sit 6-8 x NAT5s in S4 (or a vocational college course), 5 x Highers in S5, then more Highers or Advanced Highers in S6. Different schools do different numbers of NAT5s, you will be limited to what your school allows. There is flexibility to do NAT5s in S5, do Highers over two years S5-S6, crash Highers in S6 etc and also some college courses you can do.
In S2 we had picked 9 subjects to study. In S4 we dropped to 7 subjects to study at NAT5 level. English, Maths and 5 others. The selection is usually limited by columns.
English - BrightRed Course Book (not the Study Guide) is really good for RUAE revision and will let you know how the course is structured.
Maths - TeeJay maths is good for practice questions.
Science - again BrightRed books are good for NAT5 and also let you know how the course is structured.
Geography - either BrightRed or Leckie books are good.
Do you know what school you are going to yet? It is possible they will have started the NAT5 content after easter break so you will have a bit of catching up to do, if you know the school they should be able to tell you if they started studying any English texts or which units they have covered in science etc and you can read up on during the summer.
In S5, assuming you did ok in your NAT5s, you drop 2 subjects and study 5 at Higher (or perhaps 3-4 Highers and a college course). You can also "crash" Highers that you haven't done before in S6.
Prelims (similar to mocks), are either late December or early January. Final exams late April-early June.
If you have specific questions let me know.