It is excellent that you are unnerved about loans. You should be. They can ruin your life. If you want to do four years of college and four years of medical school, you really can't do it all on loans. You need a way to pay for some of it without loans.
You might look into military/Coast Guard. I believe there is a way to go to a medical school through the military. You might also look at college in three years (using credits from AP classes, high SAT scores, full schedules, etc.).
There's a book "How to pay for college without going broke" by Chany (Khany?) that's good to explain the options.
One of the best things you could do is be a National Merit Scholar (by taking the PSAT in the high school junior year and scoring extremely well). Scholars often are offered full tuition scholarships.
Anyway, to answer your question, you usually start paying loans six months after you leave school on one type of loan. On other types of loans you may have to start paying within a few months of the loan starting. The details on the loan you take are important to know. Many people "defer" payment by staying in school or by filling out deferment paperwork, but that can add thousands to the total repayment costs.
Read the Chany book. It will help. Good luck