For GCSEs I'd advise working hard and taking them seriously. The majority of people don't treat GCSEs as important, try and ignore those classmates and get the highest grades you can. If you have a subject you struggle with, work at it - don't just accept that your bad at it. Try to avoid B grades or less.
For A level, I found the teachers put more pressure on us. Try not to let this get you down. By now you should know how much work you need to do to get the grades you want. But still work hard.
In terms of sacrifices, you need to spend a lot of time on your application. Work experience and volunteering can take up a lot of time, but so does confirming these places.
My biggest piece of advice though would be, since your still young, is to keep an open mind. At 18 I'm still not sure medicine is 100% for me, I don't think anybody can be that sure. Always research other careers, be aware of all the options available to you. It would be awful for you to realise medicine is not for you and not have a clue what else is out there.
Whatever you go on to do, good luck!