I see./ I understand./ Yes, sir.
알겠습니다(al get sseum ni na) has three meanings in common usage. One is to politely express that you understand what the person is trying to say, as is done with the English phrase "I see." The second is used when concluding a discussion to show that you understand the subject that has been discussed, as in 'I understand." The last use is to give confirmation that you have understood and will carry out another person's request as in "Yes, Sir." When replying to customers or senior staff in a company, you can use this expression. In closer relationships, it is better to use the more friendly from 알겠어요.
I don't understand./ I don't know./ I'm not sure.
잘 모르겠는데요(jal mo reu get neun de yo) also has three meanings. One is to politely express that you don't understand the subject that has been discussed, as in the English "I don't understand." The other is to explain that you do not have the concrete information that the person is asking about, as in "I don't know." The last use is to explain that you do not know something for sure, as in "I'm not sure." This expression is only used in colloquial conversation.
No, I'm not very good at it.
When complicated on one of your abilities such as speaking a foreign language or cooking, you can respond in a humble manner by saying 아니요, 잘 못해요(ani yo, jal mot ha yo). In Korea, being humble and modest with regard to one's ability has traditionally been considered polite and courteous. So even if you know that your performance really excels, it is important that you do not express overly positive opinions about yourself.
I think so too.
When agreeing with another person's opinion in colloquial conversation, you can say 저도 그렇게 생각해요(jeodo greot ke saeng gak he yo). If you are in disagreement, you can express disagreement indirectly using 그렇게 생각하세요?(greot ke saeng gak ha se yo?)(Do you think so?)
Let's do that.
When giving your approval on the final proposal after discussing a certain subject, you can say 그러시죠(geu ru si jo). It is more often used by men with their co-workers than by women. It will sound formal, so in close relationships you can use 그래요(geu re yo) instead. This form is more commonly used by women.