It's your choice, but there's nothing wrong with dressing like 'a girl' from time to another. My guess is that bạn don't like wearing a lot of make up either? But if bạn really want to look thêm girlish, I can help you. First, bạn need to do your hair (I don't know what your hair looks like, so I can't really give bạn advice).
Then bạn can start wearing heels. Start with the low ones (6,5-7cm) and then, when bạn can walk straight in them, change to high heels (10-12cm). That will make bạn thêm feminine, and boys will be all over bạn (trust me).
bạn also need to have a thêm feminine style. That means no baggy pants and no comfortable clothes. At summer (this really depends where bạn live) bạn can wear a nice, but simple summerdress. To buy new clothes, bạn need to know what season bạn are, and what body shape you've got.
link is a really helpful site.
If bạn don't like dresses, bạn can always wear short skirts hoặc shorts. There's a lot of things bạn need to know if bạn wanna follow the fashion, so my Lời khuyên is finding your own style first, and then mix your style with what's trendy.
My last Lời khuyên is to find a role model. Mine is Kate Moss, she is THE guru of what's hot. Just look at the picture. She is actually wearing rubber boots, shorts and a vest. But she still does manage to look fabulous! Another one is Miley Cyrus (whether bạn like her hoặc not, she's got an amazing style).
EDIT: Oh, and I forgot the most important thing: Accessories. Match them, but avoid mathcing them with your clothes. bạn could for example wear a simple white summer dress with a brown belt, brown pumps and a brown bag. That's a simple and cheap combination.