When you lived at home, your parents probably decided when you needed to see a doctor, and one of them would take you. They'd take your prescriptions to the drugstore, and maybe make you chicken soup, too.

But now you're on your own, in college or your first apartment, starting your first job or looking for one, making your own decisions. When you get sick, you're the only one around to take your temperature - if you own a thermometer. You have to decide if you need to go to the doctor, and get yourself there - if you know one to call. Chicken soup? Open a can.You're in the confusing, sometimes difficult transition to adulthood, and part of that is learning to become an independent medical consumer and to take care of yourself when you're sick.

But how?

If you're a college student, the obvious place to go for medical help is the college health service. These services are vastly improved over college infirmaries of decades past. Dr. Norman P. Spack, a specialist in adolescent and young adult medicine at Children's Hospital and in private practice in Chestnut Hill, Mass., says there's been a revolution in university health services in the last 25 years.

Once a haven for older male doctors who took on the work in their retirement, college health services now are commonly staffed by doctors and nurses who have a special interest in young adults and are much more attuned to women's health concerns. That's especially important because young women seek medical care far more often than young men do.

This more enlightened care is matched by what specialists see as a much- heightened interest among students in health and preventive care.

College health centers now offer a host of preventive medical services, including cholesterol and blood pressure screening, routine gynecological care, advice on diets and eating disorders, counseling on the stresses of college life and guidance in becoming an independent health consumer.

At Boston University, for instance, students are given a brochure telling them what to do in case of illness and the number for a tape-recorded phone-in service that provides information on about 73 illnesses.

At Harvard, new students are being assigned to a nurse-practitioner and physician, in hopes they will form a relationship with this team as their primary care-givers.

"It's an educational process," says Dr. David S. Rosenthal, director of Harvard Health Services. Many students have never been to a doctor without a parent and don't realize the importance of getting to know one who, in turn, will know them and their medical histories.

Rosenthal says one of his major goals is teaching students to find and use primary caregivers; students too often come to the health service's urgent- care clinic demanding an immediate fix for non-urgent problems. The result is long waits for care, delivered by a rotating staff of doctors and nurses who don't have the chance to get to know their patients.

When you strike out on your own, finding and getting to know a primary care doctor is a crucial part of becoming medically independent, says Spack. That's harder when you're not in college, when you must find doctors and medical insurance without the structure of a pre-paid university health service.

It's even harder if you wait until you're sick. Spack recommends asking friends for recommendations. If you know any medical students or medical trainees, ask their opinions about doctors with whom they've worked.

Other options include community health centers, reproductive health clinics and, as a last resort, hospital emergency rooms. In Boston, neighborhood health centers are listed in the blue pages of the telephone book under Boston Department of Health and Hospitals.

Then, make an appointment for a first office visit, if not a full physical. That way, when you're running a fever of 103 degrees and have an awful sore throat, you'll know whom to call, and the doctor will know you. She'll be more likely to fit you into a busy schedule if she knows you, Spack says.

Another good route for finding a doctor, Spack says, is to call a local university health service and ask for names of doctors to whom they refer patients. Hospital emergency room personnel can be excellent sources of referrals, too, he says.

Once you've found a doctor and determined how you'll pay for visits or have them covered by insurance, ask what hospital the doctor uses; if it's not one you'd want to use, find another doctor.

If you belong to a health maintenance organization, inquire about the plan's arrangements for selecting a primary doctor.

Young women are much more likely to have a doctor than young men, in part because they need contraception and annual Pap smears to detect early signs of cervical cancer. But if their doctors are gynecologists who won't treat a sore throat or abdominal pain, Spack says, they may be just as isolated, medically, as people who don't have a doctor at all.

If young people don't always have doctors, however, it's not because they don't care about their health. It's common wisdom that young people don't think about illness because they feel immortal, but several specialists said that young adults today, particularly more affluent ones, are intensely interested in adopting healthy lifestyles.

College students now want their cholesterol tested and many maintain low- fat or vegetarian diets. Few smoke now, and drinking has declined, too, say Rosenthal and Dr. Julius W. Taylor, director of the BU Student Health Service.

Says Taylor: "It's the single biggest thing we've noticed . . . There's been a swing toward preventive medicine.

"Kids used to be very carefree about junk food. They thought all this stuff was something to worry about at 50 or 60. Now, they see that if you don't do the right thing now - well, I had a kid say `I'm killing myself right now. My cholesterol is over 200. I want to do something about it."'

Young adults who are poor, on the other hand, Spack says, have far less reason to feel that their health choices will matter in the long run. These young people, understandably, think more about the murder rate in their neighborhood than the saturated fat in their food.

Even without that kind of fear, though, the transition to adulthood can be a scary, depressing, anxious time. A good primary care physician or your college health service should be able to suggest sources for counseling to help with these feelings.

When you're seeing your doctor, he or she should inquire not only about your medical history but your lifestyle habits. Leading causes of death among 15-to-24 year-olds are car crashes, murder and suicide. - When to call the doctor:

To call or not to call? Dr. Norman P. Spack, a specialist in the care of adolescents and young adults, offers this list of some (not all) symptoms and situations that should prompt you to see a doctor:

-Abdominal pain that gets worse, especially with a fever.

-High fever.

-Very sore throat, especially with fever and swollen glands, or sore throat in which you can see white patches.

-Fever associated with rashes.

-Blood in urine or stool.

-Vomiting or diarrhea severe enough or prolonged enough that you can't keep fluids down.

-Unexpected vaginal bleeding.

-Menstrual period two weeks late (or get a pregnancy test).