If, after much gentle trying over a period of time, your anus just won't loosen, I would suggest you might have a mental wish not to be entered that you aren't aware of. If this might be, explore the possibility in your mind and with your partner; you may want to see a counselor or sexologist, or you may decide that anal intercourse just isn't for you.

If you do progress in your explorations, the time will come for your partner to insert his penis. If this is what you both want, let it happen as it will, without planning on doing it. Be easy about it, trying one of the positions I've described. It may take several (or many) tries, so relax and feel the sensations. If it hurts, and it might, just ask him to withdraw gently. Some pain may happen, and this is usually normal, just as in vaginal sex can be painful at first. If it's a strong or sharp pain, back off, but you'll discover that the mild pain turns to blissful delight during sex. As he enters, you may experience a violent urge to go to the bathroom, or you may imagine you're going to piss or shit right there. This is a fantasy of your mind and body, through lack of use and conditioning; if you respect these feelings and have patience, they will change through practice.