High School Senior probably going to nontarget - What can I do to prepare

once you start college, join relevant clubs, start networking early and apply to diversity programs if you qualify for those

Since you have time before you even start college - take time to understand what investment bankers do and if you really want to do it. If you decide you really want to do ib you will be ahead and if not then you will be in good shape too. 

a) Read the WSJ every day and try to form intelligent opinions about the markets. b) Hustle for a finance-related internship for after your senior year by cold emailing every possible small i-bank/investment manager/ finance startup you can (probably will be unpaid). c) Enjoy being a senior, you're only in HS for 1 more year!

Honestly bro, this entire forum is primarily non-targets prepping for finance careers, so this topic has been covered extensively. I recommend doing a search and looking at existing threads, and if you end up having more specific questions then you should make a thread on that. Not trying to be a dick, just saying this is a pretty frequent topic and there's a lot of helpful info out there already. 

I would say try and take accounting early and try and learn the information best as possible! And then pair that with the rest of the advice above.

Go to community college. Get your AA, build your resume and join clubs and extracurriculars to showcase teamwork and leadership. Get a 4.0 or as close to one as you can get. Apply for a transfer to a target school. Then do what all the target kids do. California community colleges have articulation agreements with UC’s and CSU’s (google: assist dot org). A lot of my peers transferred to UC Berkeley, Stanford, and UCLA. Think of community college as a second chance at high school only this time you can do it with a goal in mind. The only thing you’ll be missing out on are networking opportunities at the target universities for the first two years and a sophomore internship, but you’re getting the same credits at a quarter of the cost, if not free since you are a CA resident. 

I go to one of those UCs. There will be a finance club, join it. Network from day one, intern at a boutique bank sophomore summer if not freshman year, learn technicals asap, get a high GPA, and hustle. Become the president of the finance club and a PM of the associated students fund. You have to be creative and take advantage of the network. Seriously, you’re gonna have to call every nearly every alum in the industry. It’s feasible, though. Good luck.

Honestly if you're still in HS who's already signed the SIR for wherever you're going, if you're set on finance set your sights on the transfer programs at Berkeley, Georgetown, Dartmouth etc. You've got an entire session to make an impression in which you both qualify yourself academically and professionally. Take advantage of that special time that is being a freshman but also be cognizant of the transfers you want which lie ahead 

Well since you’re going to a nontarget, you  can start by beginning to actually give a fuck