Select the files you converted via "File--Import" and perform your editing.
When it's time to export, select "File--Export--To Media." For YouTube and most other Flash-based Web video, the preset for YouTube widescreen HD is best.
Select the "Video" tab, scroll down, and select "Render at Maximum Depth." Also select "Use Maximum Render Quality."
Finally, crop the video with the crop tool. Make sure to set the crop proportions to something that's appropriate for the video you're working with.
Select the appropriate aspect ratio for the motion picture size. When in doubt, crop. The first number is width, the second height. 16:9 is a standard size appropriate for YouTube. 4:3 is more appropriate for standard-definition television sets.
Click the "Output" tab to verify your work before you encode the video.
Verify your export settings and destination. Adobe Premiere defaults to exporting to the desktop. Once verified, press the "Queue" button.
The Export function renders the movie with Adobe Media Encoder, which is designed to run in the background but will still use a fair amount of system resources.