1. From Arduino Reference Manual--
Figure-1:
2. Description
The toInt() method extract a numerical value from a string of digits; where, the variable myString of Step-1 must be containing a string of digits from 0 to 9.
3. Example
void setup()
{
Serial.begin(9600);
String myString = "1234";
int z = myString.toInt();
Serial.println(z, DEC); //shows: 1234
}
void loop()
{
}