Not all women can have vaginal orgasms. Even if you can have a vaginal orgasm, not every sex position or sexual partner can make you orgasm this way. Physical changes during your cycle can further complicate matters.
Can you orgasm from just fingering yourself inside your vagina (or using a dildo) when you masturbate? If not, the chances are very small that you will be able to do this with a partner.
If you are meaning to ask how you can orgasm during intercourse, you will have to find a way to stimulate your clitoris during intercourse. This can mean that you or your partner needs to rub your clitoris while he is inside you or you will have to find a position where you can grind your clitoris on his pubic bone between strokes. Woman on top and doggy is ideal positions that will allow for easy access to your clitoris for both you and your partner during intercourse.
You can also search the interwebs for Coital Alignment Technique (C.A.T.). It is however an advanced position so be warned that it will not work with a new or casual partner or without some discipline and practice.
Happy hunting!!