Medicine, Dentistry, Vets
Most medical & dental schools require you to have some relevant work experience; because it’s so competitive this is now almost compulsory. Without it, you will significantly reduce your chances. This should be in a health related field or any ‘caring’ setting.
It is not easy to secure work experience in a hospital, shadowing doctors, due to concerns about patient confidentiality, however you should try. Write to all your local hospitals (there is often a work experience manager) as well as your local GP.