Audio's the most important part of video quality. The very first thing you should upgrade should be your audio. The key to good audio quality is to have the mic as close to your mouth as possible. A $100 mic will sound better 10cm from your mouth than a $5000 mic 100cm from your mouth.
Level 1 ($20-40) - Start with a cheap lavalier microphone and plug it into your phone / camera. This video explains how to nicely and gives some good recommendations. Lav mics are great for when you're moving around (eg: my apartment tour video) or when you're on a super tight budget.
Level 2 ($50-70) - Graduate to a shotgun mic like the Rode Videomicro, and use any cheap microphone stand to get it as close to your mouth as possible while staying out of frame. Do what this guy does. You can use a mic extension cable (like this one) to plug it into your phone / camera, or you can use an external video recorder (eg: Tascam DR05) and sync audio in post ("in post" means "during post production", which means "during your editing phase"). Check out my Skillshare class on editing for more about how to sync audio (it's super easy).
Level 2.5 ($250) - You can replace your Rode Videomicro with something like the Rode Videomic Pro+. That's what I used for over a year. You can still use your standard thin mic cable to plug it straight into your camera, or into an external recorder. I was using this microphone setup from the start of my channel until June 2019 (250k subs).
When you're ready to upgrade your audio, the next level involves getting an XLR microphone. All the "pro" mics use XLR (a thick audio cable) rather than the tiny thin microphone ones. The issue is that most non-cinema cameras don't have XLR audio input, so you're going to have to record audio separately.
Good price-quality ratio ($250) - Rode NTG4+ ($230) - What I've been using since June 2019.