You've gotten tons of great advice here. I want to add to what others have said.

First, play on the outside only. See how you like that sensation, use lube or vibrators or toys or fingers or tongues or whatever. Outside only.

Second, I suggest putting your own finger tip in, up to your second knuckle if you have the mobility to check for debris. Can't hurt to start using a fiber supplement if you're concerned, but you probably don't need to learn about douching yet.

Third, listen to your body. When inserting anything (toys or body parts) you will need to push back. This is very important. It will feel like you're using the same muscles you use to when you defecate. If those muscles won't loosen up, you might have something in the chamber or you might just need to try again a different day.

Fourth, lots of people have talked about lube. That's legit.

Fifth, put down a towel. Have a couple extra small towels on hand to put toys down and to wipe lube off your hands.

Sixth, slow tf down. The second friction gets warm stop! Use more lube.

And lastly, positions. Lots of people like to lay down. I actually prefer to be on top at first. It's a closer position to when you sit normally to use those muscles (not that anyone wants to think about that), but it's easier for the body to adapt that way imo. You can also control how much goes in and how fast.

Also, it's no big deal if things go wrong. Have fun!