Always film horizontally.
Use a tripod or dolly for increased stability.
Vary your shots, and remember that too much movement can be a bad thing.
Give your people enough headspace.
Ignore your photographer instincts (but only in some ways).
Consider how you want to edit footage while shooting.
Google tripods and there should be some cheap options for starting out.
Also remember the competition.
Helpful articles: https://www.nytimes.com/2018/04/17/smarter-living/beginners-guide-phone-video.html