"I am going upstairs."
Translation:Idę na górę.
If you were walking up a hill, you could say "Idę do góry". If you're walking up the stairs, you probably have in mind ending up on a specific floor. "Góra" here is simply a higher floor. For example, my house has 3 floors: parter (ground floor), pierwsze piętro, drugie piętro (first floor, second floor). If I'm at the ground floor and I say "Idę na górę", it means that I'm going either to the first or the second floor. But I won't use "Idę do góry", because that would be weirdly descriptive, instead of the normal=figurative meaning of 'góra'.
EDIT: It seems that in some regions, "Idę do góry" can be used in the context of stairs as well. I would still consider only the "na górę" version a standard one, though.