Wow. Having your first bottoming experience with a guy with a big dick probably isn't ideal, but it certainly can work.
Falme boy has some great advice, follow it.
First off, mentally focus on him and all those things about him that turn you on. Don't obsess over his cock. Love him for him and want him becasue of who he is not what he has. It will help you out mentally in dealing with his size.
Second, you are in control. Remember that!!
Third, take it slow and not all at once for God's sake! A few inches may be all you can take the first few times. Over time you may be able to take more, but it will take time.
Fourth, work with a dildo,or even seeral (each gradually larger) to help getting used to things of that size in you. Have him work the dildo sometimes, so you can get used to being in the receptive position.
Fifth, for God's sake engage in safe sex, especially with a penis that large. It WILL cause some tearing even if he is really gentle. So a condom is even more mandatory than normal (not that one should ever not use one).
Sixth, enough desire on your part + enough patience on his part ought to make this work.
Please remember to have fun with it, even if it isn't a success. If he has tried really hard to be patient and make it work for you and it doesn't work out at first remember that he might be even more frustrated than you and possibly even a little ashamed. (When I would hurt my high school girlfriend I generally felt terrible about it and at times even hated the fact that I had such girth.)
Best of luck!