More Problems:
char estado = 'LOW';
'LOW' is not a character. If you want to assign it the defined value of LOW, remove the single quotes. Same problem down in your loop.
while(counter < 5, counter++){
counter < 5 will never get run because it will constantly be returning the result of counter++. The ',' doesn't mean do this ever time. If you want to increment counter on every loop, put it in the body, or better yet, use a for loop. In this case, however, a while loop would be better used with Serial.available().
picmirc:
My goal is the "serial read" read a number with 5 digits, sends them to a variable (string) and a counter field of string through. If the field is "1", the light comes on, if it is "0" deletes the light.
Serial.available() will tell you how many chars are available to read. A good idea would be to make sure you have no less than 5 before starting to read everything into your array.
picmirc:
I tryed with char caractere[5], now the error is: "incompatible types in assigment of 'int' to 'char [5]'" in the line " caracter = Serial.read();"
That's because as I said above, Serial.read() returns a SINGLE character, not everything that you sent to it. Look at some of the examples for Serial communication to see how it handles reading serial data.