Okay, I am going to answer this one because I was taking 10 inches when I was 14 years old - yes, I was a whore, but he was only 16.
The best position to be in is on your back with him holding your legs up. Put the head of his penis in your anus and make sure that the head is well lubed. Allow him to penetrate you with only the head of his penis until you are used to it.
Once you are used to the head, allow him to put as much of his penis into you as you are comfortable with, but keep your legs up and your buttock spread. Once you are comfortable with full penetration, then you should be able to pursue other positions.
Don't get greedy and try to take the whole thing at once, that can be a real pain in the ass. Sorry, someone had to say it lol.