Will they accept me once I change?

You sound like a smart girl and you realise image isn't everything but you would like to change a bit. The most important thing will be to start being more assertive. If someone treats you badly then speak up. We teach people how to treat us to a certain extent. As for your clothes, maybe have a look through magazines and tear out clothes and colours and looks that you'd like to try. It doesn't have to be a dramatic change but you can have fun making a mood board full of images and words that appeal to you. At your age it's normal to want to try out new things so just have fun and don't worry about other people passing judgement as it is of no concern to you. Have fun.