dont know that okcupid site but most online dating sites are garbage.

anyway, girls are shallow so if theres a picture of her just tell her how pretty she is and youd like to get to know her or compliment whatever she wrote in her profile and say you share the same interests (wich you actually should if you msg her lol)
and dont be so nervous about it, if she doesnt answer... whatever, im sure theres plenty more around who will