People do this sort of sound shifting all the time without even being aware of it. In Korean, this sound shifting takes place under some very well-defined rules. Here's one of the more important rules:
At the beginning of a word (the initial position) or at the end of a word (the final position), this letter: sounds like the k in kiss.
However, when you put it between two vowels (the medial position), it changes to sound like the letter g in guess. This is a general rule, and there are sub-rules and exceptions, which I'm not covering here. (See the details)
This means that , meaning baby, is pronounced a-gi , not a-ki, and , meaning who, is pronounced nu-gu .
Please note: The new rules for Romanization of Korean, issued by the Korean government, say that this letter should be written as “g” when it appears at the beginning of a word, even though it sounds like “k.” Focus on the written form and the pronunciation, not the Romanization.
When you get to the grammar section that covers verbs, you'll be introduced to another significant sound shift.