Go to the left hand bottom corner, and click on Media Browser.
Use this to find and import your video clip. You can drag and drop it or open it with the folder settings.
Also import the image you are using for the background.
You can see the video image on the right part of the screen, as well as the Timeline panel, where you can edit your video and add effects.
To cut out a piece of your video use the Cut tool (C) to separate the unwanted part from the rest of the video. Then select the unwanted clip with the Selection tool (V) and click Delete on the keyboard.
In the timeline panel, you can slide the red arrows to cut your video image and audio as needed.
In the bottom left hand panel, click Effects from the top menu.
Choose the Video Effects folder > Keying > Ultra Key.
Drag the Ultra Key onto your clip in the Timeline.
Click Effect Controls in the top left-hand panel, and Ultra Key should be visible.
Select the eye dropper icon.
In the panel that has your video image, select the green background.
If not all of the green is selected in your image, choose the Aggressive option in the Ultra Key settings.
In your Timeline panel, drag so that your video clip is in the middle track.
Drag your new background image to the track beneath the video.
Make sure that the video and image bars are the same length in the timeline panel.
Press the space bar to make the video play. Do the audio and images match up?
In Premiere Pro, hit File -> Export -> Media
On the right in the Export Settings window, Look at the Format option
Select Format to H.264.
H.264 will create an MP4 video file type that is playable on most computers, tablets, and smartphones.