Getting a blow-job is not really a big thing and giving a blow-job is not a big thing for a girl either. Most girls are more comfortable from a physical point of view to have oral sex than to have intercourse.
However, BJs are a bride that once you cross, it's hard to get back, for a girl. It is associated with promiscious behavior, therefore, the YES / NO balance in her mind is really "will he see me as a slut / will he see me as a girl who simply wants to make him happy".
Therefore, the best tips for getting a blowjob are:
* Hygiene. You don't know how important it is.
* Create comfort and engage either into a 69 or do cunnilingus on her side.
* Slowly push her towards your penis in order to get the idea.
* If she says "I don't do something like this", don't confront her or get mad. Instead say something "I know baby that it may be something new for you. However, our purpose here is to both feel as good as possible and in order to do that, it's better not to have any taboos, right?"
Usually when a girl feels comfortable and she respects you (as in she sees you as an equal or better), she will engage in oral sex.