What can I do about this?

There is another girl I like(yes,I like many girls).I met her almost 1 year ago(September 2008).At first we were at the same class,but later I changed class.She doesn’t even know me(she knows me as a person,maybe she knows my name/surname too.How can I approach her and what to do next?Generally,I want advice about this:how to tell a girl I like(if I know her)and how to meet girls I know(but they don’t know me)and later tell them I like them.Oh,and the girl I mentioned earlier is 16(years old,just like me).There is only one problem.Some people say she is a … hooker because she was almost always with boys(this also happened with another girl mentioned on earlier question).HELP ME,please!