How can I split a string into multiple parts?
An array is 0 indexed, which means that number 5 is out of index. "Hello" should be index valued 0-4 and not 0-5. 

Here is a better code to try out: 

void setup() {
    Serial.begin(9600);
}

void loop() {

String recivedMessage = "";

// Read
while (Serial.available())
{
    recivedMessage += Serial.read();
}

// Print debug
Serial.print("recivedMessage: ");
Serial.println(recivedMessage);


//// Uncomment the row under if you are not sure that the splitting is working
// recivedMessage = "Hello";

// Split
char first  = recivedMessage.substring(0, 1)[0];    // H
char second = recivedMessage.substring(1, 2)[0];    // E
char third  = recivedMessage.substring(2, 3)[0];    // L
char forth  = recivedMessage.substring(3, 4)[0];    // L
char fifth  = recivedMessage.substring(4)[0];       // 0

// Print debug
Serial.print("Cut message: ");
Serial.println(first);
Serial.println(second);
Serial.println(third);
Serial.println(forth);
Serial.println(fifth);


}