it isn't that black and white. are you attractive to many people physically? if not, not many girls are going to be jumping up and down to be with you and they will reject you. Get a makeover. Go to a salon or stylist and ask them to help clean up your look. google some things online to see if you can find consultations and makeovers at salons nearby. some people are beyond help, meaning even if you do clean up your look you may just be what many women see as ugly. still, you should try to improve. go to the dermatologist if you have acne or skin problems. see a nutritionist and go to a gym if you're not in shape, get some help with your wardrobe and style if need be.
people skills. this is essential. you need charisma, charm, a sense of humor. know when to laugh, what kinds of jokes to make, how to flirt, how to compliment girls, how to converse and make yourself interesting without being overwhelming or a weirdo, etc. body language too. a lot of so called nice guys are kind of nerds and geeks. no sense of how to carry themselves in a way that attracts women at all. get help in this area. be a better communicator. these are skills that will benefit you much more in general, communication with people, than simply relying on being a jerk or nice guy.
do you have money? if you're completely broke that's fine, but understand the types of girls you are not going to get. I'm not saying any kind of girl is better than the other because that's not true. I'm just saying there's a reality here. are you going for girls who are so good looking that they have the option of getting with guys who have money, jobs, cars, good looks, etc. then you need to rise to the competition if its that important to you. especially if the girl has money and a nice car too. being broke is not attractive, but some girls won't care. still, it helps in the types of clothes you can buy, upkeep in appearance, what kind of dates you can take her on, etc.
also how are you approaching these girls? are they people you have gotten to know a little bit (best option)? or complete strangers? if you're lacking as much as you must be lacking if you are constantly rejected, then you should hold off on approaching complete strangers if your confidence is so weak that you are giving up already.
thats the next thing, confidence. being a jerk isn't what most girls want, though some girls do. jerks have confidence. or at least they portray themselves that way. they seem the most confident to many girls, and the funniest, and things like that.
Being funny will get you a long way with girls I think. of course, I mean a humor that is palpable to the general population and not your weird humor that few people are likely to get.
which brings me to my last point. where are you meeting these girls? look at the things you are interested in, places you like to hang out, and look for girls there. if you go after the same types in same places, and seen no results, then change something. or try e-dating.