I need help starting my homework for Computer Science. The first bits of directions are below. Once I get started, I'll probably be able to go from there but I just need some help at the beginning.

Your program will take a user supplied sentence and double encrypt it. 1. Your program will prompt the user for a single sentence. 2. The program will then scan for the entire sentence and store it in a valid String object. a) Assume this sentence can contain any of the letters. This includes both uppercase and lowercase variants. b) Also assume that it can contain a space, any of the 10 digits and the following punctuation: a comma, a period an exclamation point and a question mark. c) Any other character would be considered invalid and should be ignored by your program. However, it should not crash your program. 3. Determine the length of the String and make a single dimensional character array (not a String array) big enough to hold every character from the String. You will probably want to consider using the String length method to help with this. Use a loop of your choosing to assign the values to the array