Talk to the girl and find out some of the things she likes to do and places she likes to go.
Bring the subject up in general conversation. Ask her what she received for her birthday in the past that she really liked. Think of gifts in terms of affordability. Purchasing a gift card from one of her favorite places is always an excellent gift idea.
Ask the girl to list her birthday wishes on a piece of paper and return it to you.
This will give you a few choices of gifts she would like to receive for her birthday. Get an idea of things she might like for her birthday from friends and family.
Participate in group conversations.
Walk up to the girl as she is talking with mutual friends and use that to your advantage to ask her what she wants for her birthday. She may feel more comfortable discussing what she wants among other peers. Make eye contact whenever possible and speak with confidence.
Approach a girl when she appears to be in a good mood.
Body language is a good indicator for when to approach a girl. Watch for her to smile or glance at you. If she waves at you, she is more than likely open to talk to you. Use the opportunity to walk up to her and ask her what she wants for her birthday.