Well, you need to start from the beginning!
You can learn online. There are dozens of sites where you can start learning the basics, but after a while (a few weeks) you will notice the classes are way too simple and not very well structured. Just google korean lessons and you'll find lots of sites.
Then you must consider buying a book. I have been using "Elementary Korean" and "Continuing Korean". I really like these books, but there are many others. Just check Amazon.com. I bought my books there.
You can post a question to get more opinions about what books are good.