Please make sure to bring enough cash or a credit or debit card with PIN number to use ATMs on the arrival day. Please note, that most shops in Germany do not accept credit cards, so you will have to withdraw money from ATMs. ATMs/currency exchange inside Frankfurt airport.
At Darmstadt central railway station there is an ATM, but no currency exchange.
Tip: Ask your home bank about possible fees you have to pay for withdrawing cash or paying with your debit or credit card!