You got two options: you train or go into it raw.
Train: with dildos and buttplugs. Ideally with butt plugs first and then gradually go bigger. With a shit ton of lube and poppers. You will have to be relaxed and push through the discomfort. Doggy (opens you up faster but more discomfort), sitting on it is less painful but you will have push yourself as you open less doing so. Eventually you will not feel pain. Then once you are somewhat use to it (after multiple multiple training sessions) and can open up on your own you can try the real thing. It will be less stressful and enjoyable.
Raw: just go for it. Ideally choose a guy with a smaller length/not so thick. With a guy that will take his time and not ramp you. Not sure if you would like to tell them its your first time but could help. Will still need a shit ton of lube on your hole and his dick. Try poopers as well. Get him to finger you or you do it right before penetration. The position like with training depends on your pain tolerance, determination and trust on the guy. If you have all those then doggy and missionary could be good. Otherwise, slowly sitting on them will be fine. Its good to remember that the pain is temporary and that once you stop clutching your anus it will feel better.
Additionally, regardless you choose to train or just do it; its good to repeat the process as soon as possible multiple times so that its less painful mentally or physically until you are use to it.