Hey,

To handle keyEvents, have a look at keyReleased() in the reference.
I think generating characters from ascii-code might be a good solution for your random words. Capital letters have an ascii-value between 65 and 90, you can easily convert integers to char, here is an example as starting-point;

1. void setup() {
     println(randomString());
   }
   
   String randomString() {
     // variable to store the string
     String word = ""; 
     // random length of the string (5-9 characters)
     int l = (int)random(5, 10); 
     // generate l random letters
     for (int i =0; i<l ; i++) {
       // append a random letter to the string (ASCII 65-90)
       word += (char)floor(random(65, 91));
     }
     return word;
   }