Write A Program That Separates A String Into 2 Substrings?

I am trying to write a program that separates a string into 2 substrings.Lets say I need the entire string to be 15 letters long.I need to break the string up so that the program reads it as two words.Lets say that the first word has to be 8 letters long and the second word has to be 6 letters long.So if I type it like this: Computer Screen

That should be 15 letters including the space between the two words.I need the program to read the string to make sure that the entire string is 15 letters long and if its 15 letter long continue, but if it is not 15 letters long stop and give me an error.If is 15 letter long next the program needs to read and make sure that the first word is 8 letters long and if it is print out the first word and continue but if not it needs to give me error.

Lastly if the first word is 8 letters long it needs to read and make sure that the second word is 6 letters long and if it is print out the second word and then continue, but if it is not give me error.The problem I'm having how to declare the first and second word and substrings, can someone please help me understand how substrings works and declare them after I have declared a string.