To unlock the Give to the Poor trophy in Red Dead Redemption 2, you need to approach the $ symbol in any camp and select Contribute and donat $250 over time, OR select the ledger and buy camp upgrades to the tune of $250 over time -- this also counts as donation.
Don't worry too much about doing this in Chapter 2, as you won't be earning huge amounts of money. You'll get plenty in Chapters 3 and 4, but make sure to finish this before you finish Chapter 4 as it won't be available after that. For reference, the end of Chapter 4 is triggered by