no offense, but i'm sure the "work experience" of a twenty-year-old is probably nothing stellar. The fact that you waitressed, baby-sat, taught swimming, or worked a cash register is probably not going to land you a killer internship. Just apply and say although it's your first job, you're excited about it and are punctual/responsible/reliable.
No worries, apply, write the essays, and rock out at the summer job. Do a good job, get your foot in the door at a hospital/med school, meet some great people, and maybe you'll even come out of it with a publication.
In the meantime, you might consider having jobs/extracurriculars to help you meet people and learn to manage your time.