Hello, well for nearly everything Maths and English are essential, other typical ones would be Science, ICT would be good for you and then just any others you think you would be able to do, it's normally how well you do not the subject that counts. I'm not sure about the differences in difficulty but I'm sure others here do. Yes you tend to do A Levels after your GCSEs. None of them a particularly hard, you seem intelligent so you shouldn't have to big a problem with them. You would need a minimum of 5 GCSEs for pretty much everything, but if you want to go to a more prestigous university you are probably going to want more, there isn't a limit but most people have aroud 8-10, but it is more important to do well in the ones you have rather then having alot. Tips are just to make sure you know the sylabus for your subjects inside out, whether thats by finding information on the internet or buying textbooks and writing notes on the information.