HELP! New partner wants a finger up the butt! 😬

Hi I’ve recently started dating this guy, we haven’t had sex yet, just a little foreplay. We had our first conversation about sex today, confessing fantasies, asking about each other’s interest and all that stuff. He told me that he likes a finger in is back door! I tried not to act shocked, but I’m freaking out! I know it’s 2019 and butt play is now normalized but idk where to start! Please help! I’m willing to try though whenever that time comes, please give me tips on “how to” anything I should be aware of ?