A variety of experiences is best.
In today's healthcare environment, each member of a care team plays a certain role, and has a unique interaction with their patients. Shadowing a professional in their desired career field helps a student understand how they will be caring for patients, which in turn helps with a student's decision to pursue a certain career-path. The amount of hours that is competitive will vary by profession; however, students that regularly integrate clinical experience into their schedules should have no problem being competitive.
While shadowing is incredibly helpful for pre-health students, it is not the only way to gain experience in healthcare. Volunteering and working at a hospital and even experiences as a patient can count as healthcare exposure. Scribing is also a popular way to gain experience.
For pre-med students, experiences in three different specialties is ideal.
How can I get started?
Some hospitals offer shadowing programs, which can be competitive themselves. Ask people that you know that work in the field, or if all else fails, look up practitioners in your area and contact them. Remember to be polite, use proper phone and email etiquette, and dress professionally.
What about Physician Assistants?
Physician Assistant applicatiions generally require a substantial amount of direct patient care. Generally, 600 - 1000 hours are required (sometimes more), so students should consult the programs they are interested in.