Your portfolio is going to be the most important thing off all. Doing Uni can be a help but even the best uni courses are lacking, or at least that is the general opinion you get from most people.
What I said about portfolios is always going to be true but Uni’s are always getting better at the same time. Just remember not to take everything they tell you as fact.
If you want to get into level design for example you are going to need to put in loads more time than just one or two modules at uni.
Try out a few different areas to see what you like and then just put loads of your spare time into it. Maybe start of slow an hour or two a day, but the sooner you get working on stuff the faster you will get a job.
I assume you will do A levels, BTEC or what ever after your GCSE’s that two years that you will have a lot of time to put into what ever you do. Wish I had back then.