just the basics really...
if you can look for a reputable breeder, even though you can find some great snakes with people who are just starting out, you are more likely to find a better snake off someone who has been doing it for a long time...
you want a snake that is feeding well and calm... and one that you think looks the most stunning...
i woudnt just buy any old snake, if you really like a particular look then save up to get it...
also have a good chat to the breeder and see if he can show you his set up and the parents... gives you an idea of what the snake might end up like and if it has been kept in suitable conditions...
hope this helps