There's a pretty good 16-page book introducing millennials (and now zoomers) to the basics of retirement savings: https://www.etf.com/docs/IfYouCan.pdf
(that's the whole book). It covers all the starting bases and would basically work as an answer to this thread.
For passive investing, I use and strongly recommend M1 Finance (basically an entire platform and brokerage built around Modern Portfolio Theory). Given that markets are generally efficient (and the statistics we have about stock-picking almost always underperforming robo-advisors or even just the S&P in the long run), aiming for adequate returns seems a lot more sensible than aiming for extraordinary returns. Staying a little ahead of inflation and a little safe from risk via a 3-fund portfolio (even something like VTI + BND + VXUS) is an easy starting place to have a good shot at a comfortable retirement.
On the retirement side, you can avoid capital gains taxes* for something like $66.6k/yr worth of investments, using your (Roth) 401k ($19.5k+employer match), Roth IRA ($6k normally or $6k Backdoor Roth + $37.5k-[401k employer match] Mega Backdoor Roth), and your triple-tax-advantaged HSA ($3.6k). Even just a maxed-out and matched 401k on (100-your age)% VTI + (your age)% BND should work as a baseline retirement strategy (while learning more and improving your approach).
The only real no-no, imo, is not taking advantage of your 401k to at least the employer match or not knowing how your 401k is getting invested. You're already on the hook for all the risk (since you've got a 401k and not a pension), so might as well exercise the control you have.
* For the appalled non-Americans, no, this isn't legalized criminal tax evasion. If you can take advantage of the whole Mega Backdoor Roth, your average income tax rate (federal + state + Social Security + Medicare) is probably above 40%. So you're taking ~$108.6k, paying $42k/yr in income taxes (HSA is pre-tax), and investing $66.6k/yr. Even with a 20 year horizon (only true for the early years) and 5% annual return, you're gaining about $110k, so you're avoiding ~$32k in taxes (i.e., you're paying about 15% in taxes for dollars you'll see decades from now, instead of 34%). Which is fine, because that's how retirement savings incentives work- you accept the long time horizon in exchange for lower taxes. You get pensions and robust social safety nets, we get... this mess where we're on the hook for everything and most of us screw it up or get screwed over. Welcome to America.
Edited 6/3/2020 22:19:49