All the university applications are handled by
UCAS but your college should go through all that when it's time.
Generally people apply at the start of their 2nd year at college, and the applications are reviewed towards the end of January. After you make your final decision you have until August to decline it (I'm pretty sure). But as I said your college will go through everything nearer the time. That's not stopping you looking around now ofcourse.
For GCSE the only subjects that are compulsory to pass are Maths and English. A level subject wise there's no compulsory subjects, but I'm pretty sure they prefer guys who have done Science, English or Maths over Media Studies and Film Studies :wink:
When you choose your A level subjects try to think of subjects that would suit you best in the field you want to specialise in i.e. If you enjoy engineering, good subjects would be Physics, Maths, and Engineering.
It's good that your very keen so early on, definitly speak to the people at the Officer Liasion centre and they will be able to help you tons more than I can. Goodluck!