If you really want to become a doctor, then that's great because it means that you know what you want to be. People used to keep telling me to become a doctor and I didn't realise it, but being a doctor is just not for me and I was being very stressed figuring out what I want to do, so this is an important decision for you to make - HOWEVER, it does not mean you have to make a decision now.
The best advice I can offer is to do everything possible that will leave you with a wide range of options. For example, if you ever get the chance to take a first aid course - do it. If you ever get the chance to be involved in helping at a parent's evening night - do it. If you ever get the chance to do some contest or whatever - do it. If you have any hobbies or something that you care about (e.g. blogging, piano, sports [for teamwork etc or just for fun] whatever) - keep doing it. But don't let these things stress you out - but I guess stress is inevitable, what I mean is don't think too much about it but DEFINITELY don't miss out on these opportunities - some of them are once-in-a-lifetime chances. Charlotte49 offered some good advice about how doing Work Experience will allow you to get a feel for the kind of work that being a doctor involves. I wouldn't get too bogged down in this "competition" business at this stage but it is good you're aware of it - all you need to do is keep the options available to you and be proactive JUST IN CASE something happens. One of the other ways of keeping your options is, as aforementioned, to do well in your GCSEs, which is something you should be focusing on. Trust me, no one's going to get 8s and 9s without working at least a few hours every day. In fact, having good study habits now will really help later on. If you find the going is getting tough, then deal with it. There are various ways of dealing with things but one of the hard things is to deal with family problems and stuff - it's not enough to just brush them aside. Things can build up. Well that's enough out of me for now...