Almost everyone needs to buy a present. Let's look at some sentences that you will need.
It's common to ask a birthday person what they want.
What do you want for your birthday?
Do you need anything?
Or, you can ask a friend of a birthday person.
What do you think John wants for his birthday?
What do you think I should get for John?
What are you getting John for his birthday?
Buying something for a girlfriend or boyfriend is a little different. If you don't have any ideas, then you can ask a sales person. Let's say you want to buy your girlfriend a piece of jewelry.
I want to buy my girlfriend a birthday present. Do you have any suggestions?
Or if you want to buy a boyfriend a tie or a shirt, you can go to a place like Banana Republic and ask them.
I'm shopping for my boyfriend's birthday. Can you help me pick out something?