It is probably stating the obvious, you will need a skill that matches the job you will do for the charity. You want to be able to assist the charities and fellow volunteers rather than be a hindrance.
The obvious skill for younger native English speakers is to teach English. If you are studying at university or have a degree, find a charity that needs a global understanding of your experience or qualification.
Medical skills are always highly desirable in all fields of medicine and mental health.
Maybe your hobby will be useful. Musical skills. Wildlife and conservation.