I just opened an Acorns account in 2020. How do I prove the amounts I’ve invested? And how do I file those investments in my 2020 taxes?

[CatinaT1]

You don't need to prove the amounts you invested.  There is nothing to include on your return until you take money out.

[Critter-3]

This is a broker's account  and you should get a tax reporting form by the end of February which will report any taxable  earnings    such as   interest, dividends, capital gains and sales you earned/made during the year.  Log into your account to see when they will send those forms.  

[Critter-3]

https://www.acorns.com/support/taxes/will-i-be-provided-with-the-proper-tax-forms/