There are two systems for text messaging in Korean, and which you use depends on the brand of cellphone you have.
Rather than having separate keys for each vowel, the Samsung system has you create the vowels by punching in combinations of •, _, and |. Take a look at this chart to see how it works.
|Korean vowel||Key sequence|
Most companies use a more straightforward system basically similar to the one used for messaging in English. Consants and vowels each have their own key, two for each key. When you look at the keypad you'll see where everything, but here's this box for reference. Press the keys in the same order you would write them out on paper.