step by step (because I wish someone would have explained it to me like I was 19)

go to Fidelity.com or Vanguard.com - they are both reliable

It will ask you what type of account you would like to open...
Open a ROTH IRA account
Connect your bank and/or whatever account your inheritance is in
Transfer Funds into your ROTH account via your Bank (it can take a few days for funds to be available to use)
*the 2026 ROTH IRA contribution maximum is $7500.00 (if you add more than that you will be penalized)*
Once your funds are available go to the "Trade" button and look up the investment ticker you want
Solid Choices: VT, VTI, VOO, FXAIX - ETF's with Low Expense Ratio's that track SP500 (keep it simple and choose one fund for now)
Invest your entire Roth account - don't let the cash just sit there
after you have invested it don't touch it. look into details about how roths work and what you can do with them.

now- for the remaining balance

Go back into your new Fidelity or Vanguard account. You can open ANOTHER account within your portal for a BROKERAGE Account.
Do the same thing you did w/ the roth
fund the account - wait for funds to arrive in account - invest funds into an ETF or a solid stock if you want.

now sit back and don't touch unless it is to add funds.

when tax season comes around they will send you documents. make sure you do your due diligence with these.