There are very small dildos that are, like, barely bigger than your index finger. Start from that, use a lot of lube, and eat good amount of fiber.

Make sure you are relaxed enough. If you clench your ass like a guillotine, you won’t enjoy it. Let it slip in gently, naturally, effortlessly. It may still feel weird at first, but then you’ll learn to do it and you’ll have plenty of fun!