Hi,
These engineering disciplines are very different and also quite wide ranging. Good advice, I think, is to follow the vocational subjects that you find you most enjoy at the moment. You don't necessarily need to have a detailed road-map of your future career, only an indication of skills that you might need for such a direction. For example, you are interested in engineering. Therefore, you might be interested in related subjects such as Physics, Electronics, Computer Science, Chemistry, Design and Technology, etc. The number of engineering disciplines available are vast, and different disciplines lend themselves to different skills. For example, if you were interested in perusing research or design involving Aerospace engineering, Physics would be an important consideration. Similarly, if you were interested in Nano Technology, a comprehensive understanding of Biology might be very useful.
In summary, I think you should peruse subjects you enjoy most/have the greatest understanding and work your way into an engineering discipline from there as opportunities present themselves to you.