You need good GCSEs (mostly A*/A grades) and the required grades for GCE A level;
GCSE: Most Medicine students will have like around 6-8 A* & rest A grades for GCSE so aim for that. Some Medical Schools have minimum GCSE requirements. They're not all the same so research on that.
GCE A level: Minimum grades for Medicine is AAAb but I'm not sure about the B for AS. The average Medicine student has 500-540 UCAS tariff points so that's like A*A*A*a, A*A*AA to A*A*A*A.