You would use the casual 괜찮아 with friends and people younger than you. A slightly more formal form is 괜찮아요. 괜찮습니다 is more formal again.
At work to my coworkers, working on the same level as me but older, I would use 괜찮아요. To kids I would say 괜찮아. To my boss I would use 괜찮습니다.
Its very easy actually. Start with the hangul chart and try writing the letters one by one, while pronouncing them at the same time. As you get better in writing, try writing english words in korean(i dint write it per se, but i still translated it inside my head). U might get it in 3-7days????