Well I would say, you don’t really have to start preparing for medical exams from class 10th. But you should concentrate on your syllabus and try to grasp the concepts. Scores won’t matter much as long as you don’t mug up things and try to understand the concepts behind each chapter, specially physics and chemistry.
Once you start your 11th std (or 10+2), you can start preparing for the medical entrances. Again I would say, whatever may the academic scores, you can always crack the entrance. I have seen students scoring just 60% and qualifying for medicals in their first attempt.
All the best! 🙂