We are living in one of the golden ages of accessibility to free painting advice and tutorials by incredible painters. Below are some links to YouTube content that will help you get started if you're a beginner, and if you're not a beginner, to develop new skills.
1. Miniac's How To Get Started Painting Playlist
I cannot recommend Miniac enough. He provides regular content, it's well-produced, it's fun to watch, and it's informative. He also brings a genuine love of the practice to what he does.
2. Sorastro's Painting Tutorials
Another incredible resource for new painters, Sorastro uses a wide variety of techniques across a wide variety of miniature brands.
3. Warhammer TV Tip of The Day
The home of St. Duncan. Even if you're not interested in Warhammer figures, Duncan & co. do a good job of explaining how to paint miniatures.
4. Miniwargamer Jay's Mini Painting 101
Up to over a hundred videos, Miniwargamer Jay's Miniature Painting 101 is an exhaustive look at everything from how to do the basics like prep a miniature, priming, laying a base coat, or sealing, to incredibly specific issues like how to paint white, and how to do specific techniques like glazing or edge highlighting.
Reddit Minipainting (/r/minipainting)
This is a really supportive community which has always been responsive when asked for advice. In addition, there's plenty of beautiful mini pictures posted on a regular basis and the posters usually are willing to tell you how they achieved their results.
Cool Mini or Not
Producers of boardgames, warehouse of the world's largest collection of mini pics, and possessors of a good forum for mini discussion. In addition, home of the Crystal Brush.
Massive Voodoo are a group of painters who run seminars and classes, who blog about their painting, and who paint incredibly beautiful miniatures, often with an explanation of how they got there. If you are a beginning painter, take a deep breath before going and do not think you're going to get those results right away, but definitely use them as an aspirational goal.
Podcasts On The Bench
On the Bench is an Australian podcast for scale modeling, but they go into detail on how to do various things that can be applied to the hobby of miniature painting. They're wiling to take the time to discuss the arcane but practical issues such as the best ways to apply decals, the most efficient pair of sprue cutters (nippers) and they also seem to appreciate that mini painting and things like Gundam are what bring people into the scale modeling hobby, so there's a minimum of "Kids These Days..." grumbling.