My sister secretly put some money in my room, and then sent me a text message telling me to study hard. How can I thank my sister?

Then listen to your sister and study hard. This is what my sister expects of you. Don't disappoint your sister's love for you, and you can live a good life and repay your sister in the future. Your sister is just instinctive, not asking you to give her anything. She just wants you to study hard and have a good future, which is what she wants to see most. If you want to thank her, try to be the person your sister wants you to be.