I get really confused on when should I use какая and when какую, aren't the same thing?
They are the same thing, in a certain sense. These are two grammatical cases of a single pronoun "какая" - "which"/"what" (a feminine form, the masculine being "какой"). Namely, "какая" is a nominative, and "какую" is an accusative.
Consequently, the form you have to choose is governed by the noun it substitutes or is attached to. For example, the nominative form is used in
Какая девушка тебе нравится?
Which girl do you like?
because the noun "девушка" (girl) is the subject in this sentence (in the Russian one) and is in nominative case itself. On the other hand, the accusative form is used in
Какую девушку ты бы пригласил на танец?
Which girl would you invite to a dance?
because the noun "девушка" is the object for the verb "пригласить (кого-то)", which requires its object to be in accusative.