I want to know how I can have an array of strings for example: a[] = {A,B,C,...,Z} and then randomly choose one character from the array (for instance 'F') and apply a font format from windows on it and then render that particular letter in the form of Bitmap image with specific width and heights pixels and display the image to the user (for example in the image box in my form).

It sound like you're trying to achieve a captcha. This link is for ASP.NET but you can easily convert it to Desktop etc.. Click Here