Hi, I try to explain for you.
好了 is usually an answer for the question such as Do you finish shower yet? You can't say 好的,but 好了 is a good answer. (you can also say我洗好了)That means when you do something and eventually you finish it, and there's someone asking you if you finish, then you can say 好了.
好的 is a much more usual answer for many questions. For example, someone asks Can you help me fix my bicycle? You can say好的. hmm.. Only if when somebody asks for a help, and you are willing to giving a hand, you could just say 好的. Actually 好的 is the same as OK in Chinese.
I hope it could be helpful for u. :)