Ok, there's a lot of well-meaning misinformation around from people who are just repeating what they've heard, so please, take it from someone with >25 years of experience of anal play of various kinds:
Not only do you not need to douche, but also, for most people most of the time, it's better if you don't. If you need to be absolutely clean as a whistle for a porn shoot or something, go for it. But otherwise, your gut is an amazing self-balancing system, that will take care of itself so long as you don't sabotage it by hosing it out. Douching can easily cause an upset gut that you'll then blame on the anal instead. So consider not doing that.
Unless you have a medical condition that precludes such, you have the ability to tell if you "need to go", based on sensations in the sphincter. If you don't have the feeling that you need to go, or at least reasonably could go, then you don't need to worry about anything; you can hop straight on that dick if you want to; it'll be fine.
If you follow the above advice, then 95% of the time it'll come out completely clean, 5% of the time there will be the tiniest amount of shit, an amount that can be cleaned in 2 seconds with a wet wipe or similar. And, you were going to clean it at least that much afterwards anyway, right?
Some further general tips:
Do: enjoy a good amount of fibre generally in your diet on a daily basis if you want things to come out clean.
Don't: make sudden drastic changes to your diet or take laxatives/anti-diarrhoea meds to try to vacate it or placate it. Once again, your gut is an amazing self-balancing system if you will kindly just let it do its thing.
Finally, some things I've generally assumed:
I assume you know about basic safe sex and can make your own decisions in that regard re what's an acceptable risk to you. I'm not going to tell you always use condoms or never do such-and-such because you know what, I'm not the boss of you. Use them or not, do things or not, it's your call.
I assume you know to use lube, and also to start small if unsure and work up. Your mileage may vary, but it is usual to discover that the difficulty of receiving anal has been greatly overstated, mostly due to prudish cishets being overrepresented in media and in life.