If you're not sure of her size, sneak into her privy drawer and check out her bra size. 36B is most common. Now that you're sure of the size and that the gift won't be returned, carefully cut the tags out of the item. Before you put the lingerie into the box, give it a misting of your or her favorite perfume. It'll come out smelling like the bedroom instead of a factory.
Some women like to wash new lingerie before wearing.
If you want her to wear it right away, you should wash it in the sink with Woolite and cold water, and hang it to dry. Don't give her the gift in public. Over a quiet dinner at home or placed on her pillow as a surprise for her to find when you're not there is best. Make sure you don't give lingerie too early in the relationship. This is an intimate, and personal gift.