What can i do to get this girl?

Guys, i am 16 years old i am a boy , theres a girl in my cl who i really like i want her to be my girlfriend,she is kinda active type not really shy(but yes sometimes) she is friendly and nice i think she knows that i like her..sometimes i talk to her, play with her.i usually make her laugh. but then sometimes when she sees me she will run away(laughing like its fun).when i ask her to sit next to me she refuses sometimes(many times i asked her for her worksheet to check answers, she would first reject then after a while ask me if i want it). after school she never says bye to me(when going back to home) usually i have to walk to her to say bye. she wouldnt send me txt messages, unless i am the first one to start the sending of messages,i have a feeling that she doesnt really like me but i really need her. i can spend time just thinking about her beautiful smile. i m not ugly nor fat. i get top positions in school and also athletics. But i m just a beginner with love. i dont know how to filrt with girls. can anyone please help me out? tell me what i should do? or tell me how i can know what she feels about me?we never hang out except i sit behind her during lesson, we are not even good firends. but i really do love her. how can i tell her that i love her? txt message? or talk to her in real life which one?and can u guys tell me any good way of proposing a girl?i mean how can i say i love you to her?many thanks buddies.