You start by making sure your project settings match your video file
The Elements Tutorial Links Page http://forums.adobe.com/thread/1275830 has links near the top about importing video and starting a project
What computer operating system is your Premiere Elements 12 running on? For now I will assume Windows 7, 8, or 8.1 64 bit.
With Premiere Elements (any version) you need to start with a project preset that matches the properties of your source media. So, that requires knowledges of the source media properties, such as
Interlaced or Progressive
Pixel Aspect Ratio
There are free properites readout programs to get this information, but often we can grab enough information by knowing the brand/model/settings for the camera recording the video.
The following contains instructions for manually setting the project preset
You should do this at this time to make sure of what is being set. The program can mess up on this at times as described in the link above.
The importance of the project preset settings goes to it directing the program to set up the correct space in the Edit area monitor for editing purposes. What did you video look like when it was in the Edit area monitor.....black borders, overblown for the monitor, other?
And, just what is this final product supposed to be? Please give details of what your choices were under Publish+Share/.
Let us pause here to review and consider before deciding what next.