I agree with Abby Garcia that you're already ahead of the game. You're a sophomore in high school, and you're thinking ahead, and that's awesome! Good for you!
Now, Abby already shared some great resources with you, so I'll take a different route in terms of answering your question.
First, seek out appropriate mentors. If you really think you'd like to go to medical school, find physicians who are either willing to mentor you, sit and chat about the path ahead of you, or allow you to volunteer or work in their offices. It doesn't matter if you do anything "medical" while in their office. Simply having exposure to the medical world will begin to give you a taste of what it's all about. You may learn you love it, or you may realize that another medical/healthcare/human service direction would serve you better (or fit your personality more closely).
Next, seek opportunities for informational interviews with individuals you admire. Offer to buy them coffee, or visit them at their place of work and bring them their favorite drink or snack. Network, and find people you can ask questions of (and make sure you're fully prepared for those interviews---and dress appropriately!)
Third, network like crazy. Meet as many people in the worlds of nursing, medicine, and health-related fields as you can. Medical school is a long and expensive journey, and the more you understand about the worlds of healthcare, the better. Like I said earlier, you're a sophomore in high school and you think you want to be a doctor. That may be what you do in the end, but maybe not. Be open, be flexible, and be willing to change your mind if your course veers in another direction. Some people are eschewing medical school and seeking a Masters or Doctorate as an Advanced Practice Nurse (APRN). These nurses can now practice independently in over 20 states in the U.S., and their schooling costs a heck of a lot less than medical school!
Fourth, do well in school, but do even more! You need super good grades to get into medical school these days, but you also need more than that. Anyone can get good grades. Volunteer at health fairs, volunteer for the Red Cross, get certified in First Aid and volunteer at marathons and sporting events. Seek CPR certification so that you have some certifications under your belt. Go to CNA school and work in a nursing home. Volunteer at the local senior center. Get involved in your community so that schools will see you as more than just a high academic achiever. You need to shine above the rest, and a robust resume of life experience will help you in that regard.
If you want to meet people, impress them, have informational interviews with important and knowledgeable people, and have a business card. Seriously! Just a simple card with your name, phone number, and email address (unless you also have a website, of course). A high school student with a business card is a rare bird, and people will remember you!
Joining Linked In and learning how to use it an expert will help you to create a robust professional network at this early stage of your life. (You may have to be 18 to join---you can find out on their site. If so, join as soon as you're 18!)
Even now, you want to be careful what you post on the Internet about yourself. Those photos of beer-soaked parties on the beach? Not so good. Anything you post online can be deleted from your account, but it still exists somewhere. Be thoughtful and careful what you post online, and keep your online presence as clean as possible. Schools and potential employers will Google you----what will they find?
Again, be flexible, open, get out there and network, and soak up as much knowledge and experience as you can.