“to get” usually means to obtain or to receive, but it can also be used to describe a change in your personal feelings, emotions, or understanding. Here are some examples: I’m going to get drunk. (future) I’m getting sick. (present) I got really hot. (past) I got a new car. (past) Oh, I get it. (present - “it” is usually an idea) I’m getting tired of this. (present) I would get a discount since I’m old. (conditional) I would’ve gotten a discount since I’m old. (conditional past) I will be getting a discount since I’m old. The sentence, “What are you going to get for your birthday?” can mean “What are you going to buy yourself for your birthday?” and can also mean “What are you going to receive from other people for your birthday?”. Usually it means the second one. If someone wants it to mean the first one they will usually be specific and say, “What are you going to get yourself for your birthday?”.

In your phrase "get something for your birthday" it means receive. But keep in mind that 'get' is a word with a lot of meanings. It can also mean to buy ("I'm getting a new pair of sunglasses at the store later today."), or even to become ("I'm getting smart" or "I got good at English this year.") Even more complex, in colloquial English, particularly in the Southern United States, it can be contracted with "to" in increasingly degenerate ways to mean "must" or "have", as in the phrase, "I have got to go to the mall" (meaning "I must go to the mall" or, more commonly, "I need to go to the mall.") which becomes "I've gotta go to the mall." It can degenerate further by losing the auxiliary verb "have" to become "I gotta go to the mall." This is, of course, grammatically incorrect, but you may hear it in some locales and dialects, especially uneducated ones. At the end of the day, 'get' is a very complex word. Hope this helps.