Most people can sit through shaky or poor video quality, as long as the sound quality is good. Most people immediately become distracted, or irritated, by wind noise, crackle, unclear dialog, or other sound problems. Treat your sound as if it is the most important thing. Control the sound in the areas that you film in as much as possible.
Owning a copy of a single song in iTunes is not the same as having distribution or use rights. When you use a song without permission, you must be able to make a case for Fair Usage. Consider the purpose and character of the use. Will you make money from your use? Consider the amount used. Do you use the entire work or a short clip? What is the effect of your use on the marketplace? This Best Practices Guide can help you make decisions.
Another good reason to be careful about copyright -- YouTube and other online video sharing platforms will sometimes block or lock videos that have long durations of popular music!
Less is more! A common mistake is to find soundtrack music that you like and use it far too loud, and far too long. Tips:
GarageBand is a digital audio program that can record and play back multiple tracks of audio, and includes many pre-made audio loops. These audio loops can be imported into iMovie to use as a soundtrack.