I USE GREEN BACKGROUND TO MAKE A SMALL VIDEO CLIP , HOW CAN I CHANGE VIDEO BACKGROUND ? AND WEAR CAN I FOUND SOME CLIPS CAN BE USED AS BACKGROUND LIKE Sands Beach,Forest,PARTY TIME ... SOMETHING LIKE THAT
THANKS .

Try this tutorial. It is done with v-lab but can be accomplished with composite lab.
Have fun,

How can I put a background behind my greenscreen footage?

That knowledgebase article might answer your question also.

As far as backdrops, you really can use whatever video you want, so any time you go to a beach or a forest or a party, record a bit of something that you could use later. Building up a library of stock footage in this way is easy and helpful, as well as being practically free.

For stuff you are unable to shoot yourself, you can try stock sites such as www.istockphotocom or www.footagefirm.com .