After creating your project and importing your files (internal link), you need to set up your sequence and timeline correctly.
After you place a clip on your timeline, sometimes you may notice that your video plays back and it's too small or maybe it looks like it's been cropped or zoomed into. That is because your sequence settings do not match the settings of your video clip. It would be as if you were placing a post card on a poster board. Or conversely, trying to place a poster inside a postcard. They are not the same size and we need to fix that.
This is an example of what you may see if your sequence is set to a large size (like a poster) but your clip is too small (like a postcard). This is how you know they don't match - your image doesn't fill the frame. (insert image)
The easiest way to fix this (but not the only way): Start with a new, fresh sequence. You can drag a video clip into the new sequence and if you get a pop-up window that asks you if you want to match the sequence settings and your clip, simply say YES and Premiere will match them for you. If you see the window below, just press the "Change sequence settings" button. (insert image)
At this point, you should be able to place clips in your timeline and it will fit the window, not showing any black space on top, bottom or the sides.
Note: If you are using multiple cameras, or sourcing video from other people, you may be working with video of differing resolutions. If you are shooting with multiple cameras, make sure you have your cameras set to the same resolution. If you are supplementing your b-roll with footage from your phone, make sure your phone is set to the same resolution as your camera (you can change the resolution of video on smartphones under camera settings).
If you find that some of your clips are too small or too large to fit the timeline frame, you can resize the clip by selecting the clip in your timeline, click on the Effect Controls tab>Scale. Do not scale UP a clip more than 20% above its original size or it will look pixellated. Scaling DOWN is ok.