You need to do some homework before -- ask her what she has been using in the past perfume-wise and what were her reactions -- likes/dislikes? We also would like some info on how she reacts to perfume-wearing people in general -- her relatives, you, passersby. That said, take her to the mall and sniff some testers on the counter together -- it will also give you an idea of what she might like in fragrances in general and you will have some bonding time together, I hope.
What is the price-range you are looking into?
What are YOUR preferences in regards with a scent on a woman?
Sorry if it all sounds like too much work to do, but these are the necessities, I am afraid. :-)