At what point are you running into problems?
Can you import the background ok? If not, it may be an issue with the format of the background you are using.
Are you having difficulty actually applying the key, to remove the background from the greenscreen clip? If so, some info about which keying filters you are using, as well as an image or clip of the footage, would be helpful in pointing you in the right direction.
Are you having difficulty getting the background onto the timeline? If so, I recommend taking the time to view these tutorials
on the basics of the interface.
Are you having problems getting the background behind the greenscreen footage after they are both on the timeline? If so, you just need to re-order the layers on the timeline. Again, this tutorial
will familiarize you with the core concepts of the program.
If your problem lies elsewhere, in something I haven't covered, let us know more specifics so we can get you sorted out.