You need to consult gynecologist or urologist. The above website will give you a general idea of your problem.
Make sure that you are well aroused and ready for your partner to enter you and also use a good lubricant on both of you.
Your partner will need to go easy so as not to tear you. You could experiment with different positions so that he does not thrust so hard toward the anus, but more near to the clitoris.
May be a good idea to get checked out like Dalubaba suggests.