What is the minimum amount one needs to give to charities for it to affect one's tax return?

If you enter cash (Money) contributions in the charitable contribution area, then in the Review, if TurboTax sees that you are taking the Standard Deduction, it will stop and ask you to enter your cash contributions up to $300 to be put on line 10b on the 1040.

Because this takes place in the Review, users don't think that TurboTax will ask for it, but if you are eligible, TurboTax will.

For 2020 taxes there is a maximum "above the line" charitable contribution limit of $300. After that, charitable contributions are a SCH A itemized deduction. Until the total of all of your SCH A itemized deductions exceed your standard deduction, it has no impact on your tax liability.