1. Shoot in HD or 4KWith more people watching YouTube videos on large screen smart TV's it is becoming essential to film your videos in HD. If you own a consumer level compact digital camera or a DSLR it will almost certainly have an HD video mode. 1080p is best but 720p is adequate. Your iPad or mobile phone probably also shoots in HD, but the YouTube app may default to uploading in lower resolution. Make sure you actually upload in D.
2. Buy a tripodThere is nothing worse than shaky video, or wonky camera angles, so if you are speaking to camera or interviewing someone use a tripod. I use a Hama tripod from Amazon which cost less than £20 (link). If you are using a mobile device you can buy a cheap adapter that will allow you to fit it to a camera tripod (link to an example).
3. Get an external microphoneThe microphone in your camera or mobile device may be OK, but it will never be as good as an external microphone. Mobile phones and iPads can accept external microphones through USB adaptors or directly into the headphone sockets. For music I use a large diaphragm condenser microphone into my iPad (video). For speaking to camera I have started using a Boya BY-M1 Lavalier Microphone (tie clip type). This was £15 from Amazon (link).
Many YouTubers use shotgun microphones. These are great for outdoor filming, but as they are further from the speaker most cameras tend to add noise through the automatic gain control. If you want to try one then the Takstar SGC-598 is an inexpensive microphone that gets good reviews.