Generally, it’s not the best idea to want premed and a top college (you’re better off at a college where you’re in the top 10-25% and no 8th grader could say “I’ll be better than 75% Harvard students” :p), but since you’re in 8th grade I’ll assume “being a doctor” is shorthand for "a job that’s socially useful, uses science and people skills, and makes decent money, " and since you have 4 years to find out all the jobs that could fill that description, I’ll focus on “getting into a highly selective college”.
First, make sure you have a solid academic background: that means 4 years each of English and History/Social science, Foreign language through level 4 or AP, Math through calculus (precalculus +AP stats if you’re not aiming for STEM), Bio/chem/physics (honors or Physics 1) + 1 or 2 more of those at the AP level, for a total of about 8 APs (big ones for a STEM student would be AP English Lang, 1 AP history, AP Calculus AB or preferably BC, AP Physics1, and either AP Chem or AP Physics C, plus if you can handle those in addition to the previous ones AP Bio, AP foreign language, AP CS P or A.
DO NOT overload: it won’t “impress” colleges – adcoms are more likely to think of an overload as “hubris”.
Your ability to balance school work, sleep, social life/friends, and various activities is more important. (No college wants an “AP robot” for instance).
So, second: explore many activities in the 8th and 9th grade. Do not think of them as “college” application activities but rather as ways for you to figure what you enjoy and could be good at. Once you’ve figured that out, throw yourself at it and try to get a leadership position by junior year. By jr year it’s okay to do fewer activities, more in depth. These can be anything - they don’t have to be school clubs, in fact a combination of school and non school based activities is best (show you are involved in your community as well as in school). Become amazing at 1 or 2 of them.
Third, be a good person. Honest and kind. Someone others want to emulate and that adults respect.