Here is an old function I used to create a random text string.
Random rand = new Random();
string pw = "";
Char chars = new Char;
int count = 0;
while (count < 6)
chars[count] = (Char)rand.Next(97, 123);
pw += chars[count];
The rand.Next(97, 123) creates a number between 97 and 123 and converts that into a lowercase letter. The digits are numbers (0-9)are 48 - 57 so if you want numbers and letters you might need to add another random function that decides whether to calculate a number or a letter.