It sounds like you have more than one problem. On one hand, you find yourself being friendly and getting in trouble for it and on the other hand, you find boys responding to your friendliness and you have other girls resenting you.
The first thing that I think you need to keep in mind is that relationships, especially at your age, when hormones are racing, can be very intricate and complicated even when you don't mean for them to be.
Girls Get Nasty When They're Jealous
In addition, girls and guys tend to find themselves competing with one another and usually the girls who are getting the attention from the guys are labeled as sluts regardless of whether or not their actions deserve to be labeled as such. Unfortunately, people can get very jealous and they often allow their jealousy to make them malicious and they say things that they don't mean.
If you feel like this is you, then I say that you have no need to change your behavior but rather just be aware of the situation and try to stay clear of those girls who are jealous.
However, having said that, if you are feeling guilty or even ashamed because you feel like your reputation has earned itself- in other words, you are a bit too friendly and outgoing, then there are a few things that you can look out for when you are around these guys and girls.
Do you find yourself at the center of attention all the time? Are you there because you are constantly trying to be there or is it just because the guys seem to be attracted to you? If you realize that you are constantly calling attention to yourself, take a step back and remind yourself that its ok not to be in the spotlight all the time.
Do You Dress Like A Slut?
If you feel that the guys just seem to be more attracted to you, ask yourself why. Is it because your clothes are too tight and your bikinis very skimpy? Or are you blessed with a pretty face and slim figure? Sometimes it is not your fault, as I have said before, that others are attracted to you.
Avoid Physical Contact
As far as general advice for NOT being flirtatious, I would say the following: Try not to use your hands and be too touchy, if you find yourself tapping people, touching their arms, pushing them gently, etc. this can often be explained as flirtatious and a lot of girls find this both threatening and aggravating.
In addition, try to be aware of your facial expressions- do you bat your eyes? tilt your head? make eyes, etc.? Once you have diagnosed your flirtatiousness, or lack thereof, then you can find a way of toning it down.
Remember, guys find quiet, friendly, mysterious girls too.
Good luck- always remember to be yourself.