First off, that isn't a good question.
It contains no detail of what you want, and makes no real sense.
Secondly, that isn't how it works. The idea is: You try to do it, you get stuck, you ask for help.
Just asking for code will not please anyone, and will result in you not getting help.
String to byte is easy, and I assume what you are trying to achive:
string s = "Hello";
byte bytes = System.Text.Encoding.ASCII.GetBytes(s);
Google will help you with encryption and decryption.