Ask the employer what the salary range is for the position (they should already have a number budgeted). Add $10-$15K to the higher number in the range they give you to get an idea of what you can push for if you are able to convince them that you will be a really remarkable hire. If you have a hard time figuring out how to do that, then you are probably not at that point in your career yet (which you shouldn't feel bad about if you're just starting out).