How much are you looking to invest per year above and beyond your existing IRA/401k/HSA?
If it's less than, say, $50,000/year I agree with the advice below. Just invest in a single simple stock fund until your taxable investments hit $100,000-$200,000 and use that time to read up on investing and portfolio diversification.
Diversification can be a misleading concept though, keep in mind that almost all the benefits come from dividing your investments among 2-4 broadly uncorrelated categories (for example domestic stocks, international stocks, and cash/bonds). Sometimes people go overboard and try to slice and dice their portfolio into 20-30 different mutual funds that are all pretty correlated with each other. If you start saying things like "I'm thinking about putting 7.5% of my portfolio in mid-cap international value ex-europe" you've gone too far.
I just plow it all into VTI and call it a day.
VTI is the ETF. If you really want to invest specifically in mutual funds the equivalent is VTSAX. Whether you buy it as an ETF or mutual fund, it invests in almost every company in the USA, has an extremely low expense ratio $4 per $10,000 invested per year, and is likely something you'll want to continue to hold throughout saving for FIRE and living FIRE, reducing the need to trigger capital gains by selling to buy other investments later.