There are several steps you must take when trying to replace something in a video. It depends on the shot and what your intent is.
Sometimes you could just replace the face. (There are several good tutorials for using Mocha to do face swapping.
However, often you need to completely replace them. That means you have to recreate the background behind the original person so that they're not there and then composite the new person.
That means matching the lighting, creating a quality green screen lighting setup and many other things. It is not something to jump into as a beginner.
I also got sometimes white dots on the screen - wonder what it is..
Could you show us a screenshot or something? I have no idea what you're talking about.
The background isn't much of a problem. It's a static shot, so you need to build a "clean plate". Go look it up to learn what it is. You can use masks, multiple layers & such in AE to make it.
Can't tell you much about the wite spots! Not enough information to even guess.
Then you have to take a good, hard look at the lighting on the current subject so you can match it. That's lighting and photography technique, and not "do something magic in AE". We can't teach you lighting in a simple forum post -- you're on your own for that.
Finally, you need a good camera to pull a photorealistic chroma key, and not some Handicam piece of crap; I presume you know the basics of shooting green screens.
And if all of this leaves you scratching your head, I can suggest that you put your aspirations on hold for a bit and begin reading. This ain't magic wand stuff. This is putting your time in to get it right, as we've ALL had to do... and you're no different:
If I knew where to start and do it step by step...all I can do is chromakey stuff a bit. clean plate or other stuff sounds to be like space..can anone just do me this movie without this girl ? PLEASE and write how was it done.
Got some advices here but I'm a noob so...CGTalk - removing character from background
As Dave said, if you're new, you really need to start from the beginning and learn the basics before trying to tackle a complex task like this.
Follow the link that he provided. If you work your way through those resources, you'll learn how to do challenging compositing tasks like this.
I'm doing it arleady, 3h a day^^ yet it will take some time and I need one of these backgrounds in no time so even if I have to pay You please do this one for me or explain how to do it;;
Simply take a still frame from the first part of the shot where the girl is in the middle and then pick another still frame when the girl moves to be left or right side of center. Mask out the girl in the second still frame and put it on top of the first one. Presto, the girl is gone. Sense the camera does not move you now have a stationary background that you can use for any actor.