1. Determine if the person you are shopping for prefers gold or silver. You can usually figure this out by paying attention to what kind of jewelry she wears. If you still feel unsure you could bring it up casually in conversation by saying something like “You know, I love that gold watch you are wearing, do you prefer gold to silver?” And when you are really in a bind, you could always go with white gold, and get the best of both.
2. Browse the locket necklace selection in the metal you chose (gold, silver or white gold), and try to find one that speaks to her personality. For example, if she wears a lot of vintage clothing and jewelry, try to pick a piece with an antique feel to it. If she wears elaborate and elegant clothing, you might go for something with an intricate design or gem stones on it.
3. Figure out what to put in the locket. Most gift-givers will opt to include a photo of themselves, or a photo of themselves with the person the gift is for. But you could also use pictures of pets, other loved ones, children, or even items of sentimental value such as a lock of hair or a small souvenir. 
4. Gift wrap your present in a small box or velvet bag, and tie it off with a bow.
5. Give her the gift, and watch the joy on her face as she opens her very special locket necklace.

Tips:

1. If you are concerned about price, check pawn shops, consignment stores and discount wholesalers
2. If you still don’t feel totally comfortable going it alone, pick out a few pieces and ask her friends or family for their thoughts.