Nervous about first time anal

So my boyfriend has talked about anal and how he likes it and wants us to try it. I also want to try because I think I would like it. Whenever he grinds against my butt I really like it but there is a big leap between that and actual anal. I want to surprise him and be ready to do it when he comes home but i am a little nervous. I don't think it will hurt too bad because he is experienced and knows what he is doing. Honestly my biggest fear is that I will poop on him. How do I prepare? What do I need to get??