Translation:What are you going to get for your birthday?
"Kommer du" is like English "Are you going" but then you add "til", which is in this case "to". So we have now "Kommer du til..." and it is "Are you going to...". The last thing is "a fa" which is an infinitive verb, like English "to get". You just don't treat this "a" as another "to", it is treated like a part of the verb. Hope this is clear :)
It's very common, and especially useful in cases where "vil" would be ambiguous.
As an experiment, you can google the following sentences and note the amount of hits each gets. Remember to add the quotation marks.
"Det vil gå bra"
"Det skal gå bra"
"Det kommer til å gå bra"
Despite being the more verbose option, "Det kommer til å gå bra" is almost up there with the others.
It's the first time I hear this form of the verb "å komme", but if you think about it, it's exactly the same use of the verb "to go" in English or even "aller" for the French speakers. It's probably less natural in English, but in French, we use this form of the verb "to go" to express immediate future very often "Que vas-tu recevoir pour ton anniversaire?"
It's not a grammatically correct sentence, as the infinitive marker should be omitted after modal auxiliary verbs such as "skal". In addition to that, "skal" deals with intent, and since the intent in this case belongs to the person giving the gift and not the speaker, it's not a word I'd use.