大家好,我的名字是海伦,今年15岁.我喜欢听歌,看书,我最喜欢的书是哈利波特并且七本都看过.我最喜欢的科目英语,因为英语相对比较简单.在未来,我认为我会成为一个老师.我的座右铭：是一次只做一件事,做到最好!

Hello everybody!my name is Helen, I'm 15 years old.my favorite things include listen to music and read books. the book I loved most is Hally Botter,and I fininshed the reading of all seven books of its series.
My favorite subject is English, because I think it is comparely simple,and interesting .As for my future,I think i'd like to be a teacher.and my motto is as follows：one time just do one thing,and try your best to be perfect.
帮你润色了一下,还加了修饰词、连接词,用了高级从句类型,希望你能采纳.