what do you mean??? how can she like you if she doesnt know you??

You need to write the question properly before we can answer it. "What would you do if you liked somebody's girlfriend very much yet she doesn't even know who you are?" Compare that with your original sentence and you'll see why your question was difficult to understand. The simple answer is, get over it and move on. I know this is easier said than done, but years from now, you'll thank yourself.

If you really really have to, politely express your interest in dating her if she becomes single, and then politely take your feelings out of her life, stay away and move on. Or just directly move on.

i dont`know her but i like you so much but she is somebody's girlfriend what can i do ??