What type of necklace should I get for my girlfriend?

We are both in highschool and of the same age. By the time it is her birthday we will have been dating over 4 months. I want to get her a necklace but I don't know what kind. She likes silver and I know she likes to have a charm on her necklaces. What type of charm should I get? Her name? My name? Just a plain heart of something that says I love you? Any other ideas?