How to fit 720p video on a 1080p container?
You can make a clip scale to the resolution of the sequence by right-clicking on it and pressing Set to frame size (For older versions of Premiere, use Scale to frame size). To set this at the standard behaviour for new clips that aren't the same resolution as the sequence, go to Preferences → General and check Default scale to frame size.

However, keep in mind that upscaling a 720p video to 1080p won't yield you any more quality, worse case, it will look worse. Also, exporting to 1080p when your video is actually only 720p is kind of tricking the viewer. If you upload an upscaled video to Youtube and flag it as 'Full HD', I will so downvote your video ... So if your source material is shot in 720p, I would highly recommend you export to 720p.