Whatever position feels best for you is the one you should use. For me, it is easier to accept penetration first if I've already had at least one orgasm. Then, with lots and lots of lube, we add fingers and toys and then a cock into the mix. And some more lube.
The best position for us, to begin, is me lying sideways on the bed with my ass all the way at the edge and him standing beside the bed, with my legs thrown over his shoulders. I like looking him in the face as he grimaces, trying to go slowly, and see the way his head tilts back when he hits me at my core. From there, we can and do go on to other positions, but that is always how we start. The angle of insertion seems best.
Go slow, use more lube than you need, then add some more, and don't be afraid to experiment.
I took it up the ass for the first time on all fours, we used a lot of lube and I kept a buttplug in for about an hour prior to him fucking me. It didnt hurt much at all, the discomfort i did get was cancelled out by the sensation of him rubbing my clit.
