How do I get the arduino to store more than 1 alphanumeric letter?

i have tried run below program displaying
exit status 1
invalid conversion 'const char*' 'char' [-fpermissive]


char = "a";
void setup() {
  serial.begin(9600);

}

void loop() {
  if(serial.available() > 0){
    = serial.read();
    serial.print("you typed");
    serial.println(a);
  }

}

code: [select]

char = 'a';
will 1 character @ time. @ least compile.

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